PARIS (AFP) - Big-serving American John Isner fired down 27 aces to stun Roger Federer and reach the quarter-finals of the Paris Masters on Thursday.

Isner won 7-6 (7/3), 3-6, 7-6 (7/5) for just his second career win over the Swiss legend and first on a hardcourt.

Federer, who went out of the tournament without dropping his own serve, said that Isner had served superbly on the big points.

"He's got the size, got the power, got the angles," the 34-year-old said.

"I thought he did very well today when he needed it. The breakers, he served great. Those are the ones he needed. Yeah, that was the difference.

"It's tough to get out of the tournament not having lost your serve, you know, but that's how it can go against John."

Federer, who won the tournament for the only time in 2011, dropped an opening set tie-break 7/3 before battling back with three games in a row to win the second 6-3.

The deciding set again saw serves on top with Federer failing to convert a rare break point at 3-3.

He then twice served successfully to stay in the match to force another tie-break.

Isner jumped out into a 4-1 lead after Federer netted an overhead backhand smash and then the American had four match points at 6-2.

Federer saved three of those, but another thumping first serve brought up the biggest upset so far in the tournament.

Isner will play David Ferrer for a semi-final spot after the Spanish eighth seed outlasted Grigor Dimitrov of Bulgaria 6-7 (4/7), 6-1, 6-4 in another third round tie.