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Venus and Serena Williams were so eager to reach the Olympics semifinals they collided and tumbled to the grass chasing a shot Thursday.
They won the point and the match, beating No. 2-seeded Sara Errani and Roberta Vinci of Italy 6-1, 6-1.
Serena was at the net and prepared to hit a shot when Venus came forward to take it.
"She said 'Mine,' and she said 'Mine' again," Serena said. "I was like, 'I don't know why I'm not stopping.'"
"I really didn't see what happened," Venus said. "But Serena told me we won the point after."
The sisters, who have yet to lose a set through three rounds, seek their third gold medal in doubles. Serena is also into the semifinals in singles.
Venus seeks a record fourth gold medal in tennis.
"We have a chance to win a medal," Serena said. "That's what it's all about."
Earlier Thursday, Serena secured her spot in the singles semis when she beat former No. 1 Caroline Wozniacki of Denmark 6-0, 6-3.
Serena, seeking the first singles medal of her career, has lost only 13 games through four rounds. She'll face top-ranked Victoria Azarenka of Belarus on Friday for a berth in the final.
Playing on sun-splashed Court 1, Serena swept the first seven games against the No. 8-seeded Wozniacki. She kept one exchange going with a desperation, off-balance left-handed shot.
"I haven't practiced it in a couple of months," she said. "I wish I could have hit it better."
Otherwise, Serena stuck to the basics. She lost only nine points on her serve, never faced a break point and hit 30 winners to seven for Wozniacki.
"I felt a little off today, believe it or not," Williams said. "But I'm really, really hard to please."
Serena Williams has won 11 consecutive matches this summer at the All England Club, including her fifth Wimbledon title a month ago.
Azarenka beat No. 7 Angelique Kerber of Germany 6-4, 7-5 under the Centre Court roof. Kerber eliminated Venus Williams on Wednesday, but Azarenka feasted on the left-hander's serve, winning more than half of those points and breaking six times.
No. 2-seeded Novak Djokovic advanced by beating No. 5 Jo-Wilfried Tsonga 6-1, 7-5. Djokovic's opponent in the semifinals will be British hopeful Andy Murray, who delighted a crowd that included Prince William and wife Kate by beating No. 11 Nicolas Almagro of Spain, 6-4, 6-1.
Murray, who is seeded No. 3, was runner-up to Roger Federer at Wimbledon last month. Djokovic won Wimbledon in 2011 and was a bronze medalist in Beijing in 2008.
Americans Bob and Mike Bryan advanced to doubles semifinals by edging Jonathan Erlich and Andy Ram of Israel, 7-6 (4), 7-6 (10). Neither team managed a service break, and the Bryans sealed the victory on their fourth match point when Erlich double-faulted into the net.
The twins, who are top-seeded, won the bronze in 2008. Both are also entered in mixed doubles.
The United States was the only nation to still have an entry in all five events of singles and doubles.