Wimbledon champion Serena Williams battled past China's Li Na 6-3, 7-6 on Tuesday to reach the second round of the Stanford Classic, where she will be joined by sister Venus, after her 6-2, 6-3 victory over Canada's Stephanie Dubois.
Top seed Serena jumped out to a 6-3, 5-2 lead behind some razor-sharp groundstrokes before the hard-hitting Li fought back to force a tiebreaker.
However, the 11-times Grand Slam champion showed more ambition in the tiebreaker and secured the contest when Li hit a backhand into the net.
"She's really good and keeps improving over time," Serena said of her Chinese opponent. "I was a little off. I'd rather be a little off now and peak during the second week of the US Open."
Second seed Venus did not have to break much of a sweat against Dubois, mixing and matching a net attack with some quick sprints from the baseline.
"It was a great match to get my rhythm, hit a lot of balls and work on my shots," said Williams, who won Stanford titles here in 2000 and 2002.
"It's all about the mentality. I'm working on my transition game. I'm not afraid to come in and use my size."
Two former Stanford champions were beaten on Tuesday, last year's winner Aleksandra Wozniak of Canada losing 6-4, 4-6, 7-5 to Daniela Hantuchova and 2007 champ Anna Chakvetadze was beaten 6-4, 5-7, 7-6 by Maria Kirilenko.
Australia's Samantha Stosur upset number six seed Dominika Cibulkova 6-4, 6-3 while fourth seed Jelena Jankovic cruised past American Angela Haynes 6-3, 6-1.
Eighth seed Marion Bartoli of France overcame Ayumi Morita of Japan 7-6, 6-3.