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Roger Federer and Serena Williams both started their Olympic singles campaigns with victories, although Federer was made to work harder than expected.

The Swiss world number one was given a stern test by Colombian Alejandro Falla - who - before winning 6-3 5-7 6-3.

American fourth seed Williams beat Jelena Jankovic of Serbia 6-3 6-1.

Belgium's Kim Clijsters, set to retire at the end of the season, overcame Roberta Vinci of Italy 6-1 6-4.

Clijsters next plays Spaniard Carla Suarez Navarro, while Williams will take on Urszula Radwanska of Poland.

"I was a little nervous in the first set," said 14-time Grand Slam winner Williams, back at the scene of her recent Wimbledon triumph.

"Once I got relaxed, I played a little better. I think I played solid. I felt the ball well. I missed a few shots, but at least I was going forward.

"I think the grass is a wee bit slippery. But you've just got to deal with it, so I'll make sure I'm really stretched out before I go out for my next round, get ready to slip and slide, whatever it takes."

Awaiting Federer in the second round is Frenchman Julien Benneteau, a player he needed five sets to beat en route to the Wimbledon title.

"It's a huge deal," said Federer about the prospect of winning his first Olympic singles gold medal. "It would be a massive highlight of my life.

"I remember the emotions I had winning the gold with my Stan [Wawrinka, in the doubles at Beijing 2008].

"I have a chance, I'm the number one player in the world so I should have a chance, at medals and hopefully gold.

"It's already the fourth Olympics for me so I will give it all I have."

Czech sixth seed Tomas Berdych, the 2010 Wimbledon runner-up, was upset 6-4 6-4 by Steve Darcis of Belgium.

Berdych was a potential quarter-final opponent for Britain's Andy Murray, who faces Stanislas Wawrinka on Sunday.