Serena was forced to a second-set tie-break by Li
Wimbledon champion Serena Williams and sister Venus progressed to round two of the Bank of the West Classic in Stanford, California.
Top seed Serena was taken to a second set tie-break but battled past China's Li Na 6-3 7-6 (8-6).
Second seed Venus made light work of Stephanie Dubois, winning 6-2 6-3.
Former Stanford champions Anna Chakvetadze and Aleksandra Wozniak lost 6-4 5-7 7-6 (7-5) and 6-4 4-6 7-5 to Maria Kirilenko and Daniela Hantuchova.
The Williams sisters are competing in their first tournament since Serena beat Venus in the final at SW19, with both now looking forward to the US Open, which starts on 31 August.
"I was a little off. I'd rather be a little off now and peak during the second week of the US Open," said 11-time Grand Slam champion Serena.
Venus, who won the Stanford title in 2000 and 2002, said she was pleased with her first game back after Wimbledon.
"It was a great match to get my rhythm, hit a lot of balls and work on my shots," she added.
"It's all about the mentality. I'm working on my transition game. I'm not afraid to come in and use my size."
In the other first-round ties, 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 (8-6) 6-3, with Russian qualifier Alla Kudryavtseva beating France's Julie Coin 6-1 7-6 (7-5).
Elsewhere Russia's Maria Kirilenko beat compatriot Anna Chakvetadze 6-4 5-7 7-6 (7-5) and Germany's Sabine Lisicki knocked out Lilia Osterloh 6-2 6-3.