Novak Djokovic survived a scare before advancing as Wimbledon finalist Agnieszka Radwanska was knocked out of the tennis event of the London Olympics.
Andy Murray, David Ferrer and Maria Sharapova advanced without any hiccups while Jo-Wilfried Tsonga was made to struggle.
Djokovic, seen as the major rival to Roger Federer in the men's singles tournament of the Olympics, was made to sweat by Italian Fabio Fognini, with Serbian eventually prevailing 6-7 (7-9), 6-2, 6-2 in a rain-interrupted match.
"It's a start. 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 the rain delay," a relieved Djokovic, who was the Serbia's flag-bearer at the opening ceremony, said.
"He 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."
Talking about playing in the Olympics, Djokovic said: "It's an incredible feeling. It's really an honour and a privilege to be part of the Olympic Games which happen every four years although we wish we would be part of the Olympic Village and feel that atmosphere beside the best athletes in the world."
Great Britain's Murray, coming back to the All-England courts after his loss to Federer in the Wimbledon final, did not show any nerves as he beat Switzerland's Stanislas Wawrinka 6-3, 6-3 in just 89 minutes.
"I was saying on Tuesday, when it was getting closer, I was saying to the guys, I was so pumped to play, I want the tournament to start tomorrow," Murray said. "There was a bit of me that was a little bit anxious, that maybe I possibly peaked a bit too soon because I played really well in practice.
"I practiced with Stan a few times last week and played really well against him. That gave me a bit of extra confidence going into the match today. But I'm really up for the tournament. I wanted to play well. I want to be involved in this event for as long as possible. I'm going to give it my best shot."
Murray said the support he received after his final loss, made a huge difference to his confidence.
"After the final, it was different to what I'd experienced before," he said. "The support from friends, family, just people I bump into in the street, from politicians, celebrities. It was overwhelming. I'm not used to that.
"I understand that sometimes in the past, it wasn't always that easy to get behind me because on the court I didn't look particularly happy. But I think during Wimbledon, I think the build-up to the tournament, I just felt different on the court. I felt like I'd grown up a bit. I felt more mature. I felt like my demeanour was better.
"The support I got after the final made a huge difference to me, to my confidence, made a difference to my practice after that loss because I wanted to get back on the practice court straightaway, back in the gym, get myself ready for this tournament because I want to do my best.
"I want to achieve everything that I can. The only way of doing that is by giving a hundred percent. The support I got after the final has made a big difference to that."
Frenchman Jo-Wilfried Tsonga was in a spot of bother before winning his opening match 6-7 (5-7), 6-4, 6-4 against Brazilian Thomaz Bellucci.
Fourth seed David Ferrer of Spain had a much more easier time against Canada's Vasek Pospisil, winning 6-4, 6-4, while Japanese 15th seed Kei Nishikori got the better of talented youngster Bernard Tomic of Australia 7-6 (7-4), 7-6 (7-4).
In the women's event, Poland's flag-bearer Radwanska suffered a shock defeat to Julia Goerges of Germany 7-5, 6-7 (5-7), 6-4. Russian Maria Sharapova, however, did not have any problems, dispatching Israel's Shahar Peer 6-2, 6-0.
Victoria Azarenka was made to wait to play her first match against Romania's Irina-Camelia Begu, as rain played spoilsport.