Serena and Venus Williams claimed straightforward victories in the Olympic tennis at Wimbledon today but world number one Victoria Azarenka was taken to three sets by Romania's Irina-Camelia Begu in her opening match.
The Belarusian briefly looked in trouble when she lost the second set and then went down an early break in the decider, but she promptly reeled off six games in a row to win 6-1 3-6 6-1 and next plays Spain's Maria Jose Martinez Sanchez.
Azarenka had been held up by rain yesterday, and it was the same story for Venus Williams, who has been battling Sjogren's syndrome but made qualifying for the Olympics her top priority.
The 32-year-old has three gold medals to her name, two in doubles with sister Serena and one in singles from Sydney in 2000, and she rolled back the years with a superb 6-3 6-1 victory over ninth seed and French Open finalist Sara Errani from Italy.
Serena played her first-round match on Saturday and wasted no time reaching the third round today, seeing off Poland's Urszula Radwanska 6-2 6-3.
Radwanska is the sister of world number two Agnieszka, who was beaten by fourth seed Serena in the final of Wimbledon three weeks ago, but the Pole could not take revenge.
In the last 16, the American will meet 13th seed Vera Zvonareva, the bronze medallist from Beijing, after the Russian saw off Italy's Francesca Schiavone 6-3 6-3.
Back in the first round, there were also wins for Germany's Sabine Lisicki and Russians Nadia Petrova and Maria Kirilenko.
Venus Williams was in buoyant mood after her victory and will hope to take the same form into her second-round clash with Canada's Aleksandra Wozniak.
The American said: "It was really great. Grass is my surface and I felt great out there. It's never easy to draw a top seed. I'm unseeded here and now I feel like I'm the number nine seed, and hopefully I'll beat another seed and become their number.
"I fought so much to be here. I know there's a lot of people happy to be here, but I'm not only happy to be here I want to do something about it too and play well for my country.
"Another gold medal would be amazing. I can't even imagine the feeling, I think my head would be too big and no one would even like me anymore. I've got to get there first. I have two chances and I'll go for it."
Serena is looking for her first singles gold medal and, were she to achieve it, she would be only the second woman, after Steffi Graf, to have won all four grand slam titles and an Olympic gold in her career.
The 30-year-old has been making sure she follows the rest of the action at the Olympics, and she said: "When I'm not playing I have the TV on watching the swimming, gymnastics. It's so intense and I love every moment."
Azarenka was pleased finally to begin her campaign and admitted to nerves as she carries Belarusian hopes in the women's singles.
"It was a long wait for me," she said. "I was playing yesterday but it got cancelled.
"Your first match on Centre Court, there are always nerves, and today I couldn't get into the match in the second set. There were too many mistakes, but I'm glad I could turn things around and finish as strong as I started.
"I get more nervous when I play for my country than when I play for myself. Hopefully the next round will be much easier."
Wimbledon semi-finalist Angelique Kerber was an easy winner, the German seventh seed leading Petra Cetkovska 6-1 3-0 when her opponent retired.