Second seed Novak Djokovic of Serbia came back from a set down to beat Italy's Fabio Fognini in the first round of the Olympic men's singles.
Fifth seed Jo-Wilfried Tsonga, from France, also needed three sets to beat Brazil's Thomaz Bellucci at Wimbledon.
Germany's Julia Goerges stunned Wimbledon finalist Agnieszka Radwanska of Poland in the women's singles.
And third seed Maria Sharapova of Russia marked her Olympic debut with a 6-2 6-0 win over Israel's Shahar Peer.
Djokovic, who carried Serbia's flag in the opening ceremony, lost a first set tie-break and also had to overcome adverse weather before winning 6-7 (7-9) 6-2 6-2.
"I haven't played an official match since Wimbledon so it took me a set and a half to really get into the rhythm and obviously the rain delay affected the game and I wasn't sharp enough after it," he said.
"Fognini made some great winners and he was a set up. Then I had to be really focused for each game, try to break him and the second and third sets were really going my way."
Frenchman Tsonga, who won 6-7 (5-7) 6-4 6-4 against Bellucci, said: "I am just happy to go through for my country. I hope I will continue like this."
Goerges, ranked 24th in the world, was delighted with her 7-5 6-7 (5-7) 6-4 victory over second seed Radwanska.
"It was my first time representing Germany at the Olympics Games and it's my first time also on Centre Court at Wimbledon," she said.
"I'm pretty satisfied with the way I played. I played pretty aggressive and served well."