NEW YORK — Even if her latest troublesome first set had finished only an hour earlier, it seemed a distant memory by the time Serena Williams smacked a cross-court forehand passing winner on the run and wound up doing the splits behind the baseline.

She leaned forward, yelled and shook both fists, victory nearly hers.

Yes, Williams knows as well as anyone that you can’t count her out, no matter the deficit, no matter how the pressure might be mounting as she closes in on completing tennis’ first true Grand Slam in 27 years.

Eight times this season at major tournaments, Williams has dropped the opening set. Eight times, she has won.

The latest comeback came in the third round of the U.S. Open on Friday night, when Williams figured out a way to deal with a tricky opponent and get her own game going before it was too late, eventually emerging to grab the last eight games for a 3-6, 7-5, 6-0 victory over American wild-card entry Bethanie Mattek-Sands.

“Getting out of it so many times definitely helps me,” Williams said. “It’s definitely not something I want to do, though. But, hey, a win’s a win, I guess.”

Sure is. Williams ran her Grand Slam record to 24-0 in 2015, and 31-0 since her last defeat at a major, at Wimbledon in June 2014.

At 5-all in the second set, Mattek-Sands was two games from completing the upset. She would not win another game.

“She’s a great closer,” Mattek-Sands said. “Always has been.”

The No. 1-ranked Williams 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 U.S. 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?

“Yes, but that’s between us,” Venus said, breaking into a wide smile. “I think it worked.”

In men’s action, No. 1 Novak Djokovic and defending champion Marin Cilic moved into the fourth round, while Rafael Nadal hoped to join them in the night’s closing match. Two top-10 players lost — No. 7 David Ferrer and No. 10 Milos Raonic — and No. 14 David Goffin, citing stomach problems, became the 13th man to retire during a match because of injury or illness.

There were moments Friday under the lights of Arthur Ashe Stadium when it appeared that Mattek-Sands, who is ranked 101st as she comes back from two hip operations, would stop Williams’ streak with a varied, attacking game.

“She went for everything. She played her style, her ‘Bethanie Mattek-Sands tennis,”’ said her husband, Justin Sands, who pointed out that Williams “is never, ever out of a match.”

One issue for Williams was that she managed to convert on 3 of 16 break points through the first two sets. In the third, though, she was 3 for 5.

She also compiled a 12-1 edge in forehand winners in that decisive set.

“Usually, when I’m down, I mean, I feel like if I’m not playing well,” Williams said, “I know I can take it to another level.”