NEW YORK — Serena Williams’s serve was broken for the first time at this year’s US Open. Twice, in fact. She dropped a set for the first time in the tournament, too, pushed to the brink by Simona Halep in the quarterfinals.

In a match filled with fantastic shotmaking and enthralling exchanges, neither player budged until Williams righted herself in the third set to emerge with a 6-2, 4-6, 6-3 victory over Halep on Wednesday night to get back to the semifinals at Flushing Meadows.

‘‘I knew that I could play a lot better, and I felt like I kind of lost my rhythm a little bit in the second set. Also, Simona started playing really well. She kept going for shots, and she did what she needed to do,’’ Williams said. ‘‘But I knew if I wanted to win this, I had to step it up in the third set.’’

Halep, the 2014 French Open runner-up who was seeded No. 5 in New York, staved off all 12 break points held by Williams in the second set and forced a third.

‘‘I wasn’t very happy about that, but I tried not to let that get me down,’’ Williams said about all the chances she let slip away. ‘‘I tried to stay positive and I knew that if I did, I could just stay in the match.’’

Indeed, Williams converted the only break chance she would get — or need — in the deciding set to go ahead 3-1, and was on her way.

She finished with 18 aces, a 50-20 edge in total winners, and won the point on 26 of her 32 trips to the net.

Not bad for someone who entered the US Open with questions about a sore right shoulder that began bothering her after she won Wimbledon in July.

Williams is bidding for her seventh title at Flushing Meadows and 23d overall at a major tournament — both would be records for the Open era, which dates to 1968.

A year ago in the semifinals, Williams’ attempt to finish off a calendar-year Grand Slam ended with a stunning loss to unseeded Roberta Vinci of Italy.

Her opponent Thursday will be 10th-seeded Karolina Pliskova of the Czech Republic, who beat the 34-year-old American’s older sister, Venus, in the fourth round, en route to the first major semifinal of her career. As good as Williams’s serve is, it’s Pliskova who leads the tour in aces in 2016.

Williams, seeded No. 1, had held serve in 37 consecutive games over the past two weeks until Halep broke to get within 3-2 in the opening set. Still, Williams breezed through the rest of that set.

Then came a pivotal segment of the second. Instead of trailing 3-0, which was nearly the case, Halep wound up ahead 3-1.

First, she hung in there to erase seven break points — yes, 7! — in one game to hold for 1-all. Then she got to 15-40 on Williams’s serve, creating two break points that Williams made vanish with three aces in a row, at 117, 109 and 122 mph.

Escape complete? Not quite. Then came two more break points, the last created by a huge Halep backhand that Williams tried in vain to get back with a desperation lefty shot. Williams’ second double-fault of the game followed, and Halep would need to save five more break points before finally cashing in on her fifth set point to send the match to set No. 3.

That’s where Williams once again asserted herself, as she so often does.

In Thursday’s other semifinal, No. 2 Angelique Kerber plays two-time US Open runner-up Caroline Wozniacki. Kerber has a chance to end Williams’s record-tying string of 186 consecutive weeks at No. 1 in the WTA rankings.

Meanwhile, Andy Murray lost his way, seven consecutive games and, eventually, his riveting five-set quarterfinal against Kei Nishikori after a loud noise from a malfunctioning sound system interrupted a key point, resulting in a do-over.

Whether or not the gong-like sound, and chair umpire Marija Cicak’s let ruling, was the reason Nishikori wound up coming back to win, 1-6, 6-4, 4-6, 6-1, 7-5, it surely will be what’s most remembered about the back-and-forth, four-hour match.

After all, Murray’s extended discussions with Cicak and another official about the unusual episode came during a stretch in which he dropped 12 of 14 points.

He went from a lead of two sets to one, plus a break point at 1-all, to ceding the fourth set and trailing, 2-0, in the fifth.

‘‘I could have won the match for sure,’’ said Murray, the No. 2 seed and 2012 champion at Flushing Meadows.

Murray acknowledged a brief dip in play, but preferred to focus on other reasons for allowing the sixth-seeded Nishikori to reach a Grand Slam semifinal for the first time since he was the US Open runner-up two years ago.

“I just didn’t hold serve enough,’’ Murray said.

The US Tennis Association said Wednesday a ‘‘digital audio sound processor’’ was at fault and would be replaced before the night session.

Nishikori next faces the winner of Wednesday night’s quarterfinal between 2009 champion Juan Martin del Potro and No. 3 seed Stan Wawrinka. The other men’s semifinal Friday is No. 1 Novak Djokovic against No. 10 Gael Monfils.