The 17-time Grand Slam champion looked like he might grind out a win, saving all six break points and fighting back from 6-2 down in the decisive tiebreaker. But the Swiss star’s resistance ended when Isner – who had 27 aces in the match – hit a looping serve to Federer’s backhand on his next match point.
“It’s tough going out of a tournament without losing your serve, but that’s what happened,” the third-seeded Federer said. “I’m not sure what I could have done differently.”
Having won his sixth title of the season and 88th of his career at the Swiss Indoors last Sunday, and after racing past Italian Andreas Seppi in just 47 minutes on Wednesday, Federer was full of confidence. Even more so given he held a 5-1 lead over Isner in their career meetings, beating him most recently in the US Open fourth round.
“At the US Open I won my tiebreaks [against Isner] and here I lost them,” said Federer.
He was nevertheless generous in praising Isner, who saved a break point in the fifth game of the third set with a clean backhand volley.
“With him it’s whether he can serve big whenever he needs most,” Federer said. “I thought he did very well today when he needed it. I thought he served great.”
Federer briefly needed treatment at the start of the second set because of a sore arm, but quickly recovered.
“I was just feeling my arm, but it didn’t affect me in the third set and it’s not serious,” he said.
The 13th-seeded Isner next faces eighth-seeded David Ferrer of Spain.
Earlier, Novak Djokovic extended his winning streak to 19 matches and advanced to the quarter-finals by beating 14th-seeded Gilles Simon 6-3, 7-5, The top-ranked Serb’s victory came with some sloppy serving, however, as the 10-time Grand Slam champion was broken five times.
He will face either Tomas Berdych or Jo-Wilfried Tsonga of France in the last eight.