WIMBLEDON, England — Roger Federer leaned on the net, exhausted but exhilarated after winning the final set 19-17 to earn his first Olympic singles medal.

"It has been a long time coming," he said.

The wait included an Olympic marathon Friday, when Federer played for 4 hours, 26 minutes to beat Juan Martin del Potro of Argentina 3-6, 7-6 (7-5), 19-17. It was the longest three-set men's match of the Open era.

"I definitely got a sense that it was something special," the top-seeded Federer said. "The deeper we went into the match, the more I thought, 'Wow, this is so cool to be part of a match like this.' "

Federer converted only two of 13 break-point chances, the second coming in the next-to-last game, and had several nervous moments. But he held serve 12 times in the final set to stay in the match.

With the comeback victory, the four-time Olympian is assured at least a silver. On Sunday, he'll play in the final against No. 3 Andy Murray of Britain, who beat No. 2 Novak Djokovic of Serbia 7-5, 7-5.

Federer and Swiss teammate Stanislas Wawrinka won the gold in doubles in 2008. But Federer had been 0 for 3 in Olympic singles, the biggest blemish on a résumé that includes a record 17 Grand Slam championships.

His latest title came at Wimbledon a month ago against Murray, who relishes the shot at a rematch on the same court.

"I hope it's a great match," Murray said, "because the way the matches went today, I think the tournament deserves a great final. I hope we can provide that."

Serena Williams also clinched her first Olympic singles medal, beating No. 1-seeded Victoria Azarenka 6-1, 6-2. On Saturday, the No. 4-seeded Williams will face first-time Olympian Maria Sharapova, who beat Russian teammate Maria Kirilenko 6-2, 6-3.

Williams teamed with sister Venus to win the gold in doubles in 2000 and 2008. They have a chance to clinch at least a silver in the semifinals Saturday.

Americans Bob and Mike Bryan are assured at least a silver after beating Julien Benneteau and Richard Gasquet of France 6-4, 6-4 in the semifinals of men's doubles.

Mike Bryan and Lisa Raymond advanced to the first Olympics mixed doubles final since 1924 by beating Del Potro and Gisela Dulko 6-2, 7-5.