Serena Williams encountered late resistance from a teen champion on Wednesday before the Wimbledon winner polished off Nicole Gibbs 6-2, 6-1 in her first match at the WTA Stanford Classic.
World number four Williams was fighting off jet lag as well as reigning US college champion Gibbs, an American ranked 403rd in the world and a junior at Stanford University, host of the $740,000 hardcourt event.
It took defending champion Williams, who enjoyed a first-round bye, only 62 minutes to advance.
"I couldn't quite believe I was playing today, but I'm healthy, I have a heartbeat, so there was no reason not to," Williams said.
"I'm sure I'll feel better after a day off. I've been waking up at 2 a.m. I was sluggish today, but it was fine to get that win over with.
"The (hot) weather is definitely different than London with all that gloom and clouds. My body is used to the conditions here."
Williams, winner of 14 career Grand Slam singles titles, will next play a Friday quarter-final against South African sixth seed Chanelle Scheepers, who beat Portugal's Michelle Larcher de Brito 6-3, 6-4.
Belgian fifth seed Yanina Wickmayer ended the British challenge for the week with her defeat of Heather Watson 5-7, 6-1, 6-4.
Williams returned to California on Monday, arriving 48 hours after winning her fifth Wimbledon title. She will return to Europe late next week to train in Paris before traveling to the London Olympics, which begin on July 27.
The 30-year-old American, saying she is in the best shape of her career, was pleased to get her job done against Gibbs.
The teen saved three match points over two games before finally going down to defeat as she played her first week in a WTA main draw.
"She played really well. She moves well and doesn't quit," Williams said of Gibbs. "That's the best quality you can have as a tennis player."
"It's good to see such Americans coming up. She played a good game and was focused all the time. I was surprised that she didn't have many nerves.
"It was not a beatdown for me. She really held her own."