American John Isner stuns No. 1 Novak Djokovic in Cincinnati quarterfinals

John Isner improved his record to 15-3 on U.S. hard courts since July with a win over Novak Djokovic in Ohio Friday. (Al Behrman/AP)

MASON, Ohio -- John Isner ended No. 1 Novak Djokovic's quest to win all nine ATP Masters 1000 titles on Friday, beating the top-ranked Serb, 7-6 (5), 3-6, 7-5 to advance to the semifinals at the Western and Southern Open.

On his way to his second career win over Djokovic, Isner served at 74 percent, and though he earned just nine aces in the match, the number of unreturnable serves he hit was easily in the double-digits. The most impressive aspect of his game was his aggression. He was quick to run around his backhand to go big on his forehand returns, a move that paid off enough times to make Djokovic think twice on his second serves.

Little separated the two in the first set until Djokovic double-faulted at 3-4 in the tiebreaker to give Isner the mini-break he needed to hold out. Djokovic saw just four break points through the match and could only convert one. After Isner dropped the second, and with Djokovic serving at 5-6, 40-15, the match looked to be headed to a third-set tiebreak when Isner rallied to get it back to deuce. He earned one match point that Djokovic saved with an ace and then earned another when Djokovic threw in another double-fault. Isner finally converted when Djokovic sent a backhand into the net and did his own version of Cam Newton's "Superman" celebration in front of a packed and partisan crowd.

"It was just so much fun to play out there," Isner said. "It's one of the reasons why I work so hard to be able to be in a situation like that and to sort of enjoy it .... Certainly one of my greatest memories as a tennis player."

Three quick thoughts on Isner's big win.

This was the best match Isner has played all year: Isner's run to his first ATP Masters 1000 final at Indian Wells last year, where he beat Djokovic 7-6 (7), 3-6, 7-6 (5) in the semifinals, made a lot of people believe that he was the immediate future of American tennis. But his run over the past four weeks on North American hard courts, where he's gone 15-3 since July, has shown a level of consistency that he's lacked over the last year and a half. After a title in Atlanta, a final in Washington D.C., and now his second career win over a reigning No. 1, Isner says his game is finally coming together after an injury-laden year. This week in Cincinnati he's defeated two top-10 players in Djokovic and Milos Raonic, and scored an equally impressive straight sets win over No. 11 Richard Gasquet.

"I started out in my first round match [and] I played well," Isner said after the win. "I feel like I've played very well in each match that I played so far. I think today was the best match I played. In order for a guy like me to beat a guy like [Djokovic], I'm going to have to play extremely well; that's what happened today. So my form, yeah, it's in a very good spot right now; my body feels good. Don't have to worry about anything there. It's good times for sure.

Djokovic is proving vulnerable on hard courts: Djokovic didn't hide his disappointment after the loss, heading straight into his post-match interview. His answers were quick and short, emphasizing simply that he played a "terrible" match. "It's disappointing that I played this way," he said. "For me, it's very disappointing."

Widely regarded as the best hard court player in the game, the reigning Australian Open champion has yet to win a hard court ATP Masters 1000 tournament this year. In that span he's lost to Juan Martin del Potro (Indian Wells), Tommy Haas (Miami), Rafael Nadal (Montreal), and now Isner. In fact, Djokovic hasn't won a title on any surface since Monte Carlo in April. It's a surprising trend given Djokovic's own lofty standards and leaves him heading into the U.S. Open trying to sort out his game mentally, tactically and physically.

"I'm going to prepare as best as I can," Djokovic said. "Of course, now I'm disappointed because I really wanted to win.  But it's sport; I'll move on."

Isner will face Juan Martin del Potro in the semifinal