World No.1 Novak Djokovic’s bid to become the first man to win all nine Masters titles was ended on Friday by big-serving American John Isner in the Cincinnati quarter-finals.

Isner stunned the top seed 7-6 (7-5) 3-6 7-5, taking the tie on a second match point.

The Serb has never won Cincinnati, losing in the finals four times, most recently in last year’s decider against Roger Federer.

Isner will face 2009 US Open winner Juan Martin del Potro, who beat qualifier Dmitry Tursunov 6-4 3-6 6-1, for a place in the final.

Del Potro revealed after his victory that he was dealing with pain in his left wrist.

The Argentine missed much of the 2010 season with wrist problems and remains cautious.

“Yes, it bothers me a little bit, but not too much.

“I’m trying to play slices and different shots before the US Open to see how different I can play against the top guys or (try) doing different things.

“I’m okay – my knee is OK too and that’s important looking forward to New York. It’s nothing dangerous. I’m always in contact with my doctor; he’s taking care of me all the time.”

The seventh seed needed one and three-quarter hours to get past veteran Russian Tursunov, with the Argentine hammering 15 aces and breaking three times.

Del Potro reached the semi-finals in 2012 when he lost to Djokovic.

“This is the last tournament for me before the US Open, so it’s a good thing for my preparations. I will try to play better than today.”

In the women’s quarter-finals, fifth-seeded Li Na of China advanced over Polish No.4 Agnieszka Radwanska in a walkover.

Serbian Jelena Jankovic, seeded 14th, beat Italian No.12 Roberta Vinci 6-0 6-4.