In a heavyweight matchup featuring two of the game's big-hitters, it was Isner who came out on top after a slugfest lasting two hours and 23 minutes to set up a meeting in the last four with Argentine Juan Martin Del Potro.
Earlier, the seventh-seeded Del Potro beat Russian qualifier Dmitry Tursunov 6-4 3-6 6-1 to kick off a mouth-watering day of quarter-final action that featured the ATP Tour's 'big four' in Djokovic, Andy Murray, Nadal and Federer.
Nadal came up against an improved Federer in the final men's match of the day at the sprawling Lindner Family Tennis Center and had to withstand huge pressure for a set and a half before emerging triumphant 5-7 6-4 6-3 in over two hours and set up a last four clash against Tomas Berdych, the Czech an earlier victor over Andy Murray.
Djokovic, a four-time runner-up on the Cincinnati hardcourts, had been eager to take the final step this year and become the first player to complete what the ATP Tour has dubbed the Career Golden Masters by winning all nine World Tour Masters titles.
"I just played a very bad match overall. Terrible match," Serb Djokovic said. "From beginning to end, except the start of the second set, I was just a different player totally.
"Today I wasn't able to capitalise on my opportunities and serves. You know, in some important moments some double faults, and that's the way it is.
"Of course now I'm disappointed because I really wanted to win. But it's sport; I'll move on."
Isner, the top-ranked American at 22 who has been in good form on hardcourt surfaces with a win in Atlanta and a runner-up finish in Washington, signalled right from the start that
Djokovic was in for a fight.
The two men set a pounding tone for the match in a 47-minute opening set that went to a tiebreak with the American prevailing 7-5.
Facing a triple set point, Djokovic was able to fight off two but on the third he Slammed a return into the net, giving Isner the early advantage to the delight of the partisan home crowd.
Djokovic stepped up the pressure in the second set, breaking
Isner at the first opportunity on the way to a 3-0 lead before levelling the match.
But Isner was eager for the fight, coming back in the third and breaking Djokovic to cap the upset.
Djokovic was able to save the first of two match points with a thundering ace but Isner would seal the match on his second chance when the world number one sent a limp backhand return into the net.
Del Potro, the 2009 U.S. Open champion, continued to sharpen his hardcourt game before heading to Flushing Meadows, surviving a nearly two-hour centre court battle with Tursunov, the first qualifier to reach the last eight in Cincinnati in 11 years.
"Trying to play slices and different shots in this tournament before the U.S. Open to see how different I can play against the top guys or doing different things," said Del Potro.
"It's the last tournament for me before the U.S. Open, so it means a good thing to close my preparations to get there."
- Sports & Recreation
- Juan Martin Del Potro
- Andy Murray
- ATP Tour