American Serena Williams returns a shot to Tsvetana Pironkova of Bulgaria during the quarterfinals of the U.S. Open on Wednesday in New York. (AP photo)

She needed both of these points, one of which included a shot she hit left-handed, to reverse a deficit that reached the scale of a set and a break after 45 minutes of her quarterfinal against Tsvetana Pironkova on a cloudy Wednesday in empty Arthur Ashe Stadium.

Williams needed both of these points during a five-game, match-altering run — along with 20 aces, her most in a match in eight years — to end up on the right side of a 4-6, 6-3, 6-2 score after more than two hours to get to the semifinals at Flushing Meadows for an 11th consecutive appearance.

“It’s not how you start, it’s how you finish, right?” Williams said.

Two more victories would allow her to claim a record-tying 24th Grand Slam singles title.

Dominic Thiem of Austria serves to Alex de Minaur of Australia during the quarterfinal round of the U.S. Open on Wednesday in New York. (AP photo)

“In the beginning, I was a little fatigued, for whatever reason,” Williams said. “Obviously, I can’t do that if I want to keep winning, so I need to figure that out.”

Today, Williams will face Victoria Azarenka, who returned to the U.S. Open semifinals for the first time since 2013 by overwhelming Elise Mertens 6-1, 6-0 on Wednesday night.

In the men’s quarterfinals Wednesday, 2019 runner-up Daniil Medvedev beat No. 10 Andrey Rublev 7-6 (6), 6-3, 7-6 (5) to return to the final four, with No. 2 Dominic Thiem against No. 21 Alex de Minaur scheduled to meet at night. Medvedev hasn’t dropped a set in the tournament.

Williams also needed a comeback and the maximum number of sets to get through the fourth round before defeating 15th-seeded Maria Sakkari 6-3, 6-7 (6), 6-3.