World No.1 Novak Djokovic was knocked out in the quarter-finals of the Western & Southern Open by American John Isner.
Isner prevailed 7-6 3-6 7-5 in two hours and 23 minutes and will take on Juan Martin del Potro in the last four of the Cincinnati event, which remains the only Masters 1000 series tournament Djokovic has yet to win.
Djokovic saved the only two break points of a serve-dominated opening set, which Isner took 7-5 in the tie-break.
The Serb broke Isner's delivery for the first time on the way to a 3-0 lead in the second set and then escaped a couple of jams on his own serve to take the match into a decider.
And it was world No.22 Isner, aided by a first-serve percentage of 83 in the third set, who came through thanks to a solitary break for his second career win over Djokovic in five meetings.
"I just played a very bad match overall. Terrible match," said Djokovic, who is a four-time runner-up in Cincinnati.
"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."
Del Potro had earlier overcame the loss of the second set before sweeping past qualifier Dmitry Tursunov 6-4 3-6 6-1.
The No 7 seed is through to the last four at the event for the second consecutive season as he continues his build-up for the US Open later this month.
The 44th-ranked Tursunov was trying to become the first qualifier to reach the tournament's semi-finals since David Wheaton in 1994.
The Russian took the second set after del Potro double-faulted to set up break point, before the Argentine volleyed a short Tursunov lob into the net, giving the qualifier a 5-3 lead which soon became 6-3.
The 2009 Flushing Meadows champion responded by breaking Tursunov in the second game of the third set on his way to a 3-0 lead. Another break gave Del Potro a 5-1 lead, and he clinched the win with an ace in a love-game.