Publications
Partners
Federer had to be at his most patient in the face of a barrage of big serves from the 6ft 9in Isner, but the world No 1 was finally able to produce just enough magic to reach the last four of the Olympics for the first time since the 2000 Games in Sydney.
The 30-year-old, bidding to win his first singles gold medal, will play Argentina's Juan Martin del Potro for a place in the final and, after winning Wimbledon last month, Federer believes his current rich vein of form already ranks as one of his most golden periods.
"I think I'm playing my best. I don't know if you can play a whole lot better," Federer said.
"You always have to keep on adapting your game, making sure you come up with a new plan, keep on playing well yourself.
"Overall I'm extremely happy. Obviously being in the medal matches, at least I'll get two shots.
"That's why I looked at this match as a finals. You can imagine the relief and happiness I feel right now."
Since winning the longest match in tennis history against Nicolas Mahut at Wimbledon in 2010, Isner has embarked on a steady climb up the world rankings and he claimed a shock win over Federer in the Davis Cup earlier this year.
Isner's massive serve kept Federer at bay for much of the match, but the 17-time Grand Slam champion eventually found a way through in the ninth game of the first set.
Isner had to hammer down two successive aces to save break points, but Federer earned a third and this time the American dragged a routine forehand well wide to surrender the first break.
Even then Federer found it hard finish off Isner and he had to save a break point himself before finally serving out the set.
With Isner firmly in the groove on serve and Federer just as obdurate on his own serve, the second set was another tense affair and it needed a tie-break to decide it.
There was little to separate them in the breaker, but Federer held his nerve and got a stroke of luck at the vital time as his return on his first match point clipped the net and bounced down on the other side and out of Isner's reach.
"You just feel bad really," Federer said of his lucky break. "But also relief because it's finally over.
"It was obviously a nail-biter at the very end. Maybe I just got really lucky with that one but I felt I hit a good return and I played well throughout."
Olympic Games tennis results on Thursday (x denotes seeding):
Men's singles
Quarter-finals
Roger Federer (SUI x1) bt John Isner (USA x10) 6-4, 7-6 (7/5) Novak Djokovic (SRB x2) bt Jo-Wilfried Tsonga (FRA x5) 6-1, 7-5
Andy Murray (GBR x3) bt Nicolas Almagro (ESP x11) 6-4, 6-1
Juan Martin del Potro (ARG x8) bt Kei Nishikori (JPN x15) 6-4, 7-6 (7/4)
Women's singles
Quarter-finals
Victoria Azarenka (BLR x1) bt Angelique Kerber (GER x7) 6-4, 7-5
Maria Sharapova (RUS x3) bt Kim Clijsters (BEL) 6-2, 7-5
Serena Williams (USA x4) bt Caroline Wozniacki (DEN x8) 6-0, 6-3
Maria Kirilenko (RUS x14) bt Petra Kvitova (CZE x6) 7-6 (7/3), 6-3
Men's doubles
Quarter-finals
Bob Bryan/Mike Bryan (USA x1) bt Jonathan Erlich/Andy Ram (ISR) 7-6 (7/4), 7-6 (12/10)
Michael Llodra/Jo-Wilfried Tsonga (FRA) bt Marcelo Melo/Bruno Soares (BRA) 6-4, 6-2
Julien Benneteau/Richard Gasquet (FRA) bt Janko Tipsarevic/Nenad Zimonjic (SRB x3) 6-4, 7-6 (7/3)
David Ferrer/Feliciano Lopez (ESP) bt Marin Cilic/Ivan Dodig (CRO) 6-4, 6-4
Women's doubles
Quarter-finals
Maria Kirilenko/Nadia Petrova (RUS x3) bt Peng Shuai/Zheng Jie (CHN) 7-5, 6-7 (7/9), 6-4
Andrea Hlavackova/Lucie Hradecka (CZE x4) bt Chuang Chia-Jung/Hsieh Su-Wei (TPE) 6-3, 6-4
Serena Williams/Venus Williams (USA) bt Sara Errani/Roberta Vinci (ITA x2) 6-1, 6-1
