John Isner came through a tense third-set tie-break to beat Roger Federer 7-6(3), 3-6, 7-6(5) at the Paris Masters.
The American secured the win on his fourth match point after two hours and 16 minutes on Friday.
Federer went down despite claiming the lone service break of the match, midway through the second set.
The third-seeded Swiss struck 13 aces and saved all six break points faced, but misfired on a forehand to drop the first set and had no answer to Isner's missile serves after that.
Federer, the 2011 Paris champion, lost the initiative in both tie-breaks on Friday. In the first-set tie-break, Isner fired a forehand winner to move ahead 3-1 before serving it out.
In the decider, the American found the critical mini-break by drawing Federer to the net and forcing the Swiss into a backhand overhead miss.
Isner will contest his 10th quarter-final of the season against David Ferrer, who topped Grigor Dimitrov 6-7(4), 6-1, 6-4 in two hours and 20 minutes, also on Friday.
