Paris - Big-serving American John Isner stunned Roger Federer to 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.
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 took a 4-1 lead after Federer netted an overhead backhand smash and a backhand into the net from the 17-time major tournament winner handed the giant American four match points.
He converted the last of those for 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.