There's no Rafa Nadal or Roger Federer... but Novak Djokovic is in brilliant form

Juan Martin del Potro, who was beaten 6-2, 7-6, 6-4 by Novak Djokovic in a US Open quarter-final of the highest quality here on Thursday night, believes the Serb will go on to retain his title this weekend.

"He's playing better and better every day and every tournament," Del Potro said. "He is the favourite to win this tournament. I saw him playing at a very high level for three hours in our match and he has the intensity to win all the matches in the tournament."

In terms of the quality of the tennis on both sides of the net, this was the match of the tournament so far. Del Potro, who has been gradually recapturing his best form since missing a year after injuring his wrist in the wake of his 2009 triumph here, attacked from the start, battering Djokovic with his huge groundstrokes, but the world No 2 held firm. Some of Djokovic's defence was breathtaking as he kept taking Del Potro's biggest blows before coming off the ropes to land his own winning punches.

The turning point was the end of the second set, which lasted 84 minutes. Del Potro made the early break, but when the Argentine served for the set at 5-4 Djokovic broke back. At 5-6 Del Potro had to save three set points in a 17-minute game that contained eight deuces. Djokovic won a remarkable 20-shot rally to go 6-3 up in the tie-break and took the set with a point that summed up his ability to turn defence into attack, hitting a running backhand winner from well behind the baseline.

Djokovic, who will play David Ferrer in today's semi-finals after the Spaniard beat Janko Tipsarevic 6-3, 6-7, 2-6, 6-3, 7-6, went on to win the third set more comfortably, but paid tribute afterwards to the quality of Del Potro's play.

"It was a straight-sets win, but it was much, much closer and tougher than the score really indicated," Djokovic said. "We played for three hours. I think that second set especially was crucial because it gave me a two-set advantage and helped me feel better coming into the third.

"It could really have gone either way. It was very long and there were a lot of important and exciting long rallies. The crowd got into it. In one way it was a lot of fun to be a part of this great match.

"I was aware of his qualities before the match. He's a big guy and has a very powerful serve. His first shot is great, especially from the forehand side. He moves really well for his size. I tried to be aggressive and get him moving. I knew the longer the rally went on I would have more chances, but I couldn't make it all the time because he was very aggressive, staying close to the line. But I managed to play a great game at 5-4 in the second set and turned things around."

Djokovic, who beat Rafael Nadal in the final 12 months ago to claim his third Grand Slam title of the year, is building an admirable record of consistency. He was playing in his 14th consecutive Grand Slam quarter-final and is now through to his 10th successive Grand Slam semi-final. This is also the sixth year in a row where he has made the last four here.

Ferrer lost to Djokovic in the Serb's first US Open semi-final in 2007. The 30-year-old Spaniard has been considered a clay-court specialist in the past, but of his four appearances in Grand Slam semi-finals this will be his third on hard courts. He has won five titles on three different surfaces this year and has made at least the quarter-finals of all four Grand Slam events in 2012. However, he has never reached a Grand Slam final and on the evidence of Djokovic's form here he will still be waiting by the close of play today.