Caroline Wozniacki stunned America's Madison Keys at the US Open to reach her first grand slam quarter-final since 2014.
Wozniacki, a former world number one, but whose progress has been wrecked by injury, continued her resurgence in Arthur Ashe Stadium by beating eighth seed Keys 6-3 6-4.
The Dane will now face Latvia's Anastasija Sevastova, the world number 48 who also enjoyed a surprise win over Britain's Johanna Konta.
Wozniacki made the final at Flushing Meadows in both 2009 and 2014 but a series of fitness problems, including an ankle injury earlier this year, have seen her plummet to 74th in the world.
After missing the French Open between first round exits at the Australian Open and Wimbledon, expectations were low ahead of the final major tournament of the year, but Wozniacki has run into form in New York.
She was too consistent for Keys, who made 33 unforced errors to her opponent's seven, before crashing out after one hour and 18 minutes.
"It's crazy to be out there again and just playing really well," Wozniacki said.
"I'm not surprised that I could make it this far because I always believe in myself and I always think that when I got on court I always believe that I'll win the match."
Wozniacki climbed to world number one in 2010 but her troubles have seen her drop outside the top 50 this year for the first time in almost a decade.
"I know it sounds bad, but honestly, at this point I'm like, I really don't care what my ranking is," Wozniacki said.
"If I'm not in the top five I feel like it's not where I want to be so whether I'm 20 in the world or 100, it doesn't matter because I'm going to play the same people anyway.
"The main thing is that when I'm on court I have to believe in myself. I know I can beat anyone.
"I think it just sucks for some of the other players who have to play me early."
Keys is considered a potential successor to Serena Williams' hegemony at the top of the women's game but the 21-year-old has struggled to replicate her season last year.
After making the semi-finals at the Australian Open and the last eight at Wimbledon in 2015, Keys has now lost in five consecutive grand slam fourth rounds.
"I think it was nerves a little bit," Keys said. "Once you're in the second week it seems a little bit closer and I feel like sometimes I definitely get ahead of myself and I start thinking how much I want it."
Last year's runner-up Roberta Vinci is also through to the quarter-finals after beating Ukraine's Lesia Tsurenko in straight sets.
Vinci pulled off one of the greatest upsets in tennis history when she knocked out Serena Williams 12 months ago, and the Italian looks in good form again.
"Great memories, it is always on my mind," Vinci said.
"I know it's tough to repeat again the same results of last year, but I'm in the quarter-final. I have won four matches here. I don't want to stop. I want to continue."
Vinci will now face Australian Open champion Angelique Kerber after the world number two beat 14th seed Petra Kvitova 6-3 7-5.
The result means Serena Williams will have to reach the final and, if it comes against Kerber, win the tournament to prevent the German taking top spot in the rankings.
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