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Roger Federer and Rafael Nadal will meet for the first time since 2017 in Indian Wells after the Spaniard defied more knee problems to defeat Karen Khachanov.
Nadal took a medical time-out after the third game of the second set to have tape applied under his right knee and looked in serious discomfort before battling to a 7-6(2) 7-6(2) victory, saving a set point to avoid being pushed to a decider.
Federer, who is looking to follow up his 100th title in Dubai last month, had a more comfortable outing in ending the run of young Pole Hubert Hurkacz 6-4 6-4.
With the last-four clash taking place on Saturday, there must be a question mark over whether Nadal will be fit enough even to start the match, and certainly whether he will be able to bring his best against Federer.
Assuming he does take to the court, Nadal will face his great rival for a 39th time, looking to improve his winning record to 24-15.
Australian Open 2019 in picturesShow all 67 1 /67Australian Open 2019 in pictures Australian Open 2019 in pictures Day One Andy Murray refused to go down without a fight against Roberto Bautista Agut.
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Australian Open 2019 in pictures Day One But the Brit dropped the first set after being broken late on.
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Australian Open 2019 in pictures Day One Britain's Dan Evans reacts during his victory over Japan's Tatsuma Ito
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Australian Open 2019 in pictures Day One Katie Boulter celebrates her victory over Ekaterina Makarova in the Australian Open first round
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Australian Open 2019 in pictures Day One Ekaterina Makarova is one of the most experienced players on the tour but was beaten by Boulter
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Australian Open 2019 in pictures Day One Britain's Heather Watson reacts after her defeat against Petra Martic.
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Australian Open 2019 in pictures Day One Watson was distraught at her early exit.
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Australian Open 2019 in pictures Day One Maria Sharapova of Russia shakes hands with Harriet Dart of Great Britain after a 6-0, 6-0 victory.
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Australian Open 2019 in pictures Day One Tomas Berdych celebrates his victory over Kyle Edmund
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Australian Open 2019 in pictures Day One Tomas Berdych shakes hands with Kyle Edmund after his Australian Open first-round victory
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Australian Open 2019 in pictures Day One Rafael Nadal followed his Australian Open victory over James Duckworth by criticising the ATP
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Australian Open 2019 in pictures Day One Roger Federer celebrates after his victory over Denis Istomin
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Australian Open 2019 in pictures Day Two Venus Williams celebrates her victory against Romania's Mihaela Buzarnescu
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Australian Open 2019 in pictures Day Two Alexander Zverev hits a return against Slovenia's Aljaz Bedene
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Australian Open 2019 in pictures Day Two Ivo Karlovic celebrates his victory against Poland's Hubert Hurkacz
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Australian Open 2019 in pictures Day Two Eugenie Bouchard will now face Serena Williams after defeating China's Peng Shuai in the first round
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Australian Open 2019 in pictures Day Two World No 1 Simona Halep celebrates her victory against Estonia's Kaia Kanepi
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Australian Open 2019 in pictures Day Three Dan Evans congratulates Roger Federer after their Australian Open second round match
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Australian Open 2019 in pictures Day Three Roger Federer celebrates after beating Dan Evans to reach the third round
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Australian Open 2019 in pictures Day Three Katie Boulter suffered defeat against Aryna Sabalenka
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Australian Open 2019 in pictures Day Four Milos Raonic of Canada celebrates defeating Stan Wawrinka of Switzerland during their second round match
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Australian Open 2019 in pictures Day Four Kei Nishikori hits a return against Croatia's Ivo Karlovic
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Australian Open 2019 in pictures Day Four Kei Nishikori defeated Ivo Karlovic in a five-set thriller.
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Australian Open 2019 in pictures Day Four Simona Halep showcased her fighting spirit to overcome Sofia Kenin
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Australian Open 2019 in pictures Day Four Serena Williams eased past Eugenie Bouchard to reach the third round.
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Australian Open 2019 in pictures Day Four Johanna Konta suffered a three-set defeat against Garbine Muguruza to become the last Briton to be eliminated in the singles
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Australian Open 2019 in pictures Day Four Novak Djokovic celebrates his victory against France's Jo-Wilfried Tsonga
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Australian Open 2019 in pictures Day Five Maria Sharapova celebrates her victory against Denmark's Caroline Wozniacki
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Australian Open 2019 in pictures Day Five Roger Federer celebrates his victory against Taylor Fritz
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Australian Open 2019 in pictures Day Five Rafael Nadal celebrates his victory against Australia's Alex de Minaur
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Australian Open 2019 in pictures Day Five Stefanos Tsitsipas beat Nikoloz Basilashvili 6-3, 3-6, 7-6, 6-4 to progress to the fourth round
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Australian Open 2019 in pictures Day Five Amanda Anisimova earned the biggest win of her career to date in beating Arnya Sabalenka
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Australian Open 2019 in pictures Day Five Ashleigh Barty celebrates her victory against Maria Sakkari
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Australian Open 2019 in pictures Day Five Maria Sharapova celebrates her victory against Denmark's Caroline Wozniacki
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Australian Open 2019 in pictures Day Six Djokovic eventually regained his composure to reach the fourth round
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Australian Open 2019 in pictures Day Six Novak Djokovic was unhappy with the arena lights being turned on earlier than usual
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Australian Open 2019 in pictures Day Six Kei Nishikori salutes the crowd after defeating Joao Sousa in the third round
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Australian Open 2019 in pictures Day Six Elina Svitolina beat Shuai Zhang to reach the fourth round
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Australian Open 2019 in pictures Day Six Serena Williams consoles a tearful Dayana Yastremska after their third-round encounter
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Australian Open 2019 in pictures Day Six Naomi Osaka beat Su-Wei Hsieh to reach the Australian Open fourth round
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Australian Open 2019 in pictures Day Seven Ashleigh Barty of Australia shakes hands with Maria Sharapova of Russia
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Australian Open 2019 in pictures Day Seven Rafael Nadal serves vs Tomas Berdych
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Australian Open 2019 in pictures Day Seven Greece's Stefanos Tsitsipas celebrates his victory against Switzerland's Roger Federer during their men's singles match on day seven of the Australian Open tennis tournament in Melbourne on January 20, 2019. (Photo by Jewel SAMAD / AFP) / -- IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE -- (Photo credit should read JEWEL SAMAD/AFP/Getty Images)
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Australian Open 2019 in pictures Day Seven Roger Federer waves to the crowd after losing to Greece's Stefanos Tsitsipas
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Australian Open 2019 in pictures Day Eight Milos Raonic celebrates defeating Alexander Zverev
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Australian Open 2019 in pictures Day Eight Alexander Zverev reacts during his match vs Milos Raonic
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Australian Open 2019 in pictures Day Eight Elina Svitolina celebrates her victory over Madison Keys
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Australian Open 2019 in pictures Day Eight Svitolina believes her WTA Finals victory has proven she can compete at the top
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Australian Open 2019 in pictures Day Eight Williams' victory sees her installed as favourite to win the Australian Open
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Australian Open 2019 in pictures Day Eight Simona Halep reacts to her fourth-round defeat
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Australian Open 2019 in pictures Day Eight Serena Williams celebrates after defeating world No 1 Simona Halep to reach the Australian Open quarter-finals
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Australian Open 2019 in pictures Day Nine Rafael Nadal celebrates his quarter-final win over Frances Tiafoe
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Australian Open 2019 in pictures Day Nine Frances Tiafoe was beaten in straight sets by the Spaniard
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Australian Open 2019 in pictures Day Nine Stefanos Tsitsipas roars in celebration after beating Roberto Bautista Agut
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Australian Open 2019 in pictures Day Nine Greek fans in Garden Square celebrate Stefanos Tsitsipas' win
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Australian Open 2019 in pictures Day Nine Petra Kvitova celebrates winning match point
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Australian Open 2019 in pictures Day Nine Home favourite Ashleigh Barty was knocked out by Petra Kvitova
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Australian Open 2019 in pictures Day Nine The emotion showed as Petra Kvitova discussed her quarter-final win over Ashleigh Barty
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Australian Open 2019 in pictures Day Nine America's Danielle Collins beat Anastasia Pavlyuchenkova 2-6, 7-5, 6-1
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Australian Open 2019 in pictures Day 10 Karolina Pliskova reacts after a point against Serena Williams
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Australian Open 2019 in pictures Day 10 Serena Williams during match against Karolina Pliskova
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Australian Open 2019 in pictures Day 10 Kei Nishikori was forced to retire hurt during the second set
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Australian Open 2019 in pictures Day 10 Djokovic benefited from his opponent’s physical problems to secure a place in the semi-finals
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Australian Open 2019 in pictures Day 10
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Australian Open 2019 in pictures Day 10
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Australian Open 2019 in pictures Day 11 Naomi Osaka of Japan celebrates match point in her Women's Semi Final match against Karolina Pliskova
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Australian Open 2019 in pictures Day 11 Rafael Nadal celebrates after winning the match against Greece's Stefanos Tsitsipas
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But recent history favours Federer, who has won all five of their matches since 2014, including in the fourth round here two years ago.
Their last meeting came in the final of the Shanghai Masters in October 2017, which Federer won 6-4 6-3.
Nadal told Amazon Prime: "I had some troubles with the knee. It was some tough moments but I was able to fight and finally find a way.
"Hopefully I will be ready (for the semi-finals), I'm going to try my best to recover and play one of these matches that I want to play."
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