Three sets triumph for Serena Williams and the Murray brothers lose in the tennis singles events.
Olympics: Roger Federer stutters into second round
While Serena produced a polished display in a 6-3, 6-1 demolition of Serbia's Jelena Jankovic, Federer was made to sweat before finally beating Colombia's Alejandro Falla 6-3, 5-7, 6-3 on Centre Court.
Federer appeared to be cruising to victory in his first match since defeating Andy Murray to win a record-equalling seventh Wimbledon crown.
But the world No 1 allowed Falla back into the match after wasting three match points in the second set and was pushed hard in the decider before sealing a tougher than expected win on his fifth match point.
Federer will face France's Julien Benneteau in the last 32 in a rematch of their third round clash at this year's Wimbledon.
"It's great coming back [after winning Wimbledon], but it's so much harder to do it again in the first round knowing that every match for me is like a final.
"I'm really relieved and hoping to keep it up," Federer said.
It was much easier for five-time Wimbledon winner Serena, who was cheered on from the players' box by US First Lady Michelle Obama.
"It was great seeing Michelle there," Serena said. "It just made me want to play better and I felt even better."
Much to the disappointment of the home crowd, however, Andy and Jamie Murray crashed out of the men's doubles after losing to Austria's Jurgen Melzer and Alexander Peya. The Scots won the first set before eventually losing to the Austrian duo 5-7, 7-6, 7-5.
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