And all won.
"What a good day for fans between me, Venus, Roger and all the other players," Serena Williams said. "It's really such a great experience."
Venus Williams waited an extra day because of rain to begin her bid for a record fourth gold medal in Olympic tennis, then defeated recent French Open runner-up Sara Errani of Italy, 6-3, 6-1.
Serena completed a July sweep of Poland's Radwanska sisters by beating Urszula in the second round, 6-2, 6-3. Federer also reached the third round, beating Julien Benneteau of France, 6-2, 6-2.
After winning in singles, the Williams sisters began a bid for their third Olympic doubles gold medal by eliminating Sorana Cirstea and Simona Halep of Romania 6-3, 6-2. They won in 2000 and 2008.
"Another gold medal would be amazing," Venus said. "I can't even imagine the feeling. I think my head would be too big, and no one would even like me anymore."
Other major champions to advance in singles on a cool, sunny day included three-time Olympian Lleyton Hewitt, top-seeded Victoria Azarenka, Kim Clijsters, Petra Kvitova, Ana Ivanovic and three-time Wimbledon runner-up Andy Roddick.
Leander Paes of India became the first tennis player to compete in six Olympics and teamed with Vishnu Vardhan for a doubles victory.
Federer and Swiss teammate Stanislas Wawrinka, who won the gold in doubles in 2008, began their bid for a repeat by rallying past Kei Nishikori and Go Soeda of Japan, 6-7 (5), 6-4, 6-4. The match ended on the verge of nightfall, with Federer serving out the final game.
"It was getting really dark," Federer said. "We could be sitting here at 5-all and coming back Tuesday to finish."
The U.S. team went 6-0, with John Isner and Varvara Lepchenko also advancing. Isner, seeded 10th, hit 15 aces and reached the third round by beating Malek Jaziri of Tunisia, 7-6 (1), 6-2. Lepchenko completed a rain-interrupted, two-day, first-round win over Veronica Cepede Royg of Paraguay, 7-5, 6-7 (6), 6-2.
Roddick's victory set up the first showdown of the tournament. As a price for being unseeded, he'll face 2011 Wimbledon champion Novak Djokovic on Tuesday.
"That's my own fault," Roddick said. "It's tough. That's obviously not an ideal situation, but I'm sure he's not thrilled about it either."
Venus Williams, also unseeded, drew a tough first-round foe in Errani, who has won four titles this year and is ranked a career-high No. 9. Williams served well, charged the net aggressively and appeared at ease on the Wimbledon grass, where she has won five of her seven Grand Slam titles.
Afterward, Williams challenged the Olympic record for references to herself in the third person.
"Today definitely seemed a lot more like classic Venus," she said. "I know she's in here, but she has a couple of things she's dealing with, so she does the best she can."
Despite being diagnosed last year with an autoimmune disease that can cause fatigue, Williams began 2012 determined to make the Olympic team. She won the gold in singles at the 2000 Games and teamed with Serena to take the gold in doubles in 2000 and 2008.
Because her opening match was delayed a day by rain, Williams will have to play six consecutive days if she reaches Saturday's final.
"I definitely expect everything to be tough on me almost nowadays," she said.
Over on Court 1, Serena's serve lacked its usual sizzle, and she was even broken once, but she still eliminated Radwanska with little drama. She defeated Radwanska's sister, Agnieszka, in the Wimbledon final this month.
Serena will next play No. 13-seeded Vera Zvonareva of Russia. Williams won when they met for the 2010 Wimbledon title.
"It's not an easy tournament, playing someone tough so soon," Williams said.
Roddick converted only two of 13 break-point chances against Klizan but dominated with his serve, as often happens at Wimbledon, where he's a three-time runner-up.
"I served really well," he said. "I would have liked to have converted more break points. But I got a couple and, thankfully, that's all I needed."
The Olympic match, delayed a day by rain, was Roddick's first in eight years. He lost in the third round at the 2004 Games and skipped Beijing in 2008.
Roddick, a dedicated Davis Cup player for the United States, took Court 2 on a sunny morning looking particularly patriotic. He wore a blue shirt, white shorts and stars-and-stripes shoes.
"Posh sneakers, Andy," shouted a spectator with a British accent.
He lost only nine points in 11 service games, and the first break point came on the final shot of the opening set, which put him ahead to stay.
Roddick won with more than just his serve, hitting several improbable shots, including a back-to-the-net lob to help win one point, and a backhand as he did a belly flop behind the baseline.