The cry came from the shadows of the Centre Court stands when Roger Federer fell behind in his Olympic semifinal.

''Not enough, Roger!'' a British spectator shouted.

And so Federer gave the crowd more - in the end, more tennis than anyone anticipated. He won the final set of an Olympic marathon 19-17 to achieve another milestone victory on his favorite stage.

Federer clinched the first singles medal of his career Friday, rallying past Juan Martin del Potro of Argentina 3-6, 7-6 (5), 19-17. At 4 hours, 26 minutes, it was the longest three-set men's match of the Open era.

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.

''I was very tense at certain times,'' Federer said. ''I was seeing myself as a loser many times during the match.''

With the comeback victory, four-time Olympian Federer is assured of at least a silver. On Sunday he'll play in the final against the winner of the second semifinal between No. 2-seeded Novak Djokovic of Serbia and No. 3 Andy Murray of Britain.

Del Potro will play for the bronze.

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

Serena Williams also clinched her first Olympic singles medal, beating No. 1-seeded Victoria Azarenka 6-1, 6-2. On Saturday, 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 medal in doubles in 2000 and 2008. They had a chance to clinch at least a silver in a semifinal match scheduled later Friday.