NEW YORK » Can’t count out Serena Williams, no matter how big the deficit, no matter how off-target her strokes, no matter how much the pressure might be mounting as she bids for a calendar-year Grand Slam.

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

The latest comeback was 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 Bethanie Mattek-Sands.

"I’m not trying to live on the edge," Williams said with a big smile.

Perhaps. Still, no one does it better. And with so much at stake, no less.

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.

She won the Australian Open on hard courts in January, the French Open on clay courts in June, and Wimbledon on grass courts in July, and now is four wins away from adding the trophy on the hard courts of the U.S. Open. Next up is a fourth-round match Sunday against yet another American, 19th-seeded Madison Keys.

Looking down the line, Williams’ quarterfinal opponent could be her older sister Venus.

There were moments Friday under the lights of Arthur Ashe Stadium when it appeared that Mattek-Sands, a wild-card entry ranked 101st, would not allow that to happen.

While Mattek-Sands won doubles titles this year at the Australian Open and French Open, this was a rare moment in the spotlight for her in singles. She had appeared in the U.S. Open 13 other times, never making it as far as the third round until this week.

But by mixing in slices with flat strokes, heading to the net when there were openings — and even sometimes when there weren’t — Mattek-Sands played a varied, attacking style that gave Williams fits at times.

Particularly on break points: Williams was able to convert only 3 of 16 through the first two sets, before going 3 of 5 in the third.

"I said, ‘You know what, Serena? Just keep going,’" said Williams, a 33-year-old American who also has a chance to become the first woman since Chris Evert in 1975-78 to win four U.S. Opens in a row. "’Keep trying, keep trying, keep trying.’"

Mattek-Sands started well, taking a 3-0 lead while grabbing 16 of the first 21 points.

Williams decidedly did not, responsible for each of the match’s first half-dozen unforced errors. That opening set ended with three consecutive miscues by Williams — a backhand wide, a swinging forehand volley long, a forehand return wide.

As excited as Mattek-Sands was, and as visibly upset as Williams was, the match could hardly be considered over at that point. Consider: Williams entered with a 7-0 record when dropping the first set of a match at a major in 2015.

It took her a while to get into the second set, though, missing out on break chance after break chance, and showing her exasperation repeatedly. After one flubbed backhand, she wheeled and looked at coach Patrick Mouratoglou in the stands. After another, she leaned over, pushing her forehead against the handle of her racket.

This did not come easily.

Even when Williams finally converted her sixth break point of the second set, screaming loudly to celebrate the 5-3 lead, she quickly double-faulted twice while serving for the set, helping Mattek-Sands break right back. Soon thereafter, it was 5-all, and Mattek-Sands was two games from one of the biggest surprises in tennis history.

She did not win another game.

Earlier Friday, Venus gathered herself after a mid-match dip to reel off the five games and beat 12th-seeded Belinda Bencic of Switzerland 6-3, 6-4.

Did Venus get any family advice before facing Bencic, who defeated Serena last month en route to the title at a hard-court event in Toronto?

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