Serena Williams revealed how Tiger Woods encouraged her to return to tennis for one last glory mission after she powered into round three of the US Open.

Golf great Woods was on his feet and rapturously celebrating as Williams beat number two seed Anett Kontaveit on Wednesday night in New York.

He had been invited to sit in Williams' player box and shared conversations with Venus Williams as they watched Serena push past Kontaveit with a dynamic display, coming through 7-6 (7-4) 2-6 6-2 on Arthur Ashe Stadium.

It was remarkable from the 40-year-old on court as she found an extra gear for the deciding set, teeing up a clash with Australian world number 46 Ajla Tomjlanovic.

Williams has firmly indicated this will be her final tournament before retirement, and a host of stars have come out to watch her in action, with former US president Bill Clinton in the crowd for her first-round match.

Woods, actress Zendaya and film director Spike Lee were among those watching the Kontaveit match, and Williams suggested afterwards that she and 15-time major champion Woods had helped each other with their respective comebacks.

At this time last year, Williams was sidelined by a leg problem sustained at Wimbledon, while Woods was recovering from the serious leg injuries he suffered in a car crash.

"He's one of the reasons I'm here, one of the main reasons I'm still playing," Williams said. "We talked a lot. He was really trying to get me motivated.

"There's a few people, but we were like, 'Okay, we can do this together'.

"It was good, because I didn't know what I wanted to do. I was just lost, so many questions. When you can rely on someone like that, I mean, my goodness, he's Tiger Woods, it was really helpful to get clarity."

What Williams is wary of at this stage is raising her own expectations.

She was charmingly dismissive of an on-court question when asked if she was surprised by how well she was performing.

As far as Williams is concerned, she has proven countless times she is the greatest player, certainly of her own era, so even with limited preparation she backs herself.

But thinking too intently about winning majors may have been costly in recent years, with Williams having lost her last four grand slam finals to remain on 23 singles titles, one short of Margaret Court's all-time record.

"I cannot think that far," when asked about the prospect of winning the tournament. "I'm having fun and I'm enjoying it. Honestly, I've had so many tough matches the last I don't know how long that I just feel like just being prepared for everyone that I play is just going to be really, really difficult. Get through those moments."

In typical Williams fashion, she has danced around the subject of retirement since writing in Vogue in early August she would be "evolving away from tennis".

She spoke of wanting to "relish these next few weeks" in the magazine article, and so far at Flushing Meadows she is savouring every moment.

Serena and Venus were due to play doubles on Thursday, again on the Ashe Stadium court, with more fanfare expected.

"I think I've mostly been kind of blocking everything out, but then at the same time I've been embracing a little bit of it, because I also want to enjoy the moment," WIlliams said.

"I just feel like I have had a big red X on my back since I won the US Open in '99. It's been there my entire career, because I won my first grand slam early in my career.

"But here it's different. I feel like I've already won, figuratively, mentally. It's just pretty awesome the things that I've done."