Serena Williams will team up with sister Venus for the Olympic title in women's doubles. They play the Czech Republic's Andrea Hlavackova and Lucie Hradecka.
"We've put ourselves in a good position," Venus said. "We'll try to go for it again tomorrow. It's amazing to do it with your sister.
"We talked for four years to be here in this position," Venus said. "It means a lot to us. The Olympics is the epic, the height of our careers."
The pair defeated the Russian team of Nadia Petrova and Maria Kirilenko, 7-5, 6-4.
The sisters won the gold in doubles in 2000 and 2008. Venus also won the gold in singles in 2000.
Serena has a chance to become tennis' first double gold medalist at an Olympics since her sister. Serena beat Maria Sharapova on Saturday to win her first singles gold and complete a career Golden Slam in singles.
"I'm really just impressed that I'm doing well and playing well," Serena said, "and I'm happy and I'm energized and I'm healthy, and I have so much room for more, and I want to do more. And that makes me really, really excited."
