Serena Williams of the US reacts during her match against Bethanie Mattek-Sands of the US at the US Open Championships tennis tournament in New York, September 4, 2015. [Photo/Agencies]

The No 1-ranked Williams, meanwhile, is trying to become the first tennis player since Steffi Graf in 1988 to win all four Grand Slam tournaments in the same season. And now that milestone is four wins away.

Williams, who is 33, was asked whether she remembers watching Graf on television back then.

"Seriously? I mean, I'm old, but come on. Geez," Williams said, rolling her eyes. "I mean ... I was 6. Get serious."

Add in her title last year in New York, and Williams is bidding for a fifth consecutive Grand Slam title and 22nd overall, which would equal Graf for the most in the professional era, which began in 1968, and second-most in history behind Margaret Court's 24.

Williams also can become the first woman since Chris Evert in 1975-78 to win four US Opens in a row.

"I don't know what to expect. I've never been on this train," Williams said, then heard her own words and cracked herself up by noting: "I love metaphors."

Next up is a fourth-round match Sunday against yet another American, 19th-seeded Madison Keys, who lost to the 33-year-old Williams in the Australian Open semifinals in January, so knows what she is up against.

"Her determination is unlike anyone else's," Keys said. "You could be watching a match, and she'd be down 6-0, 5-0, 40-love, and you still don't think she's going to lose. You think she's going to come back and win."

Get past Keys, and Williams' quarterfinal opponent could be older sister Venus, who reeled off the last five games to beat 12th-seeded Belinda Bencic of Switzerland 6-3, 6-4.

Did Venus get any family advice before facing Bencic, who is responsible for one of Serena's two losses in 53 matches in 2015?