Serena Williams fought back from a set and a break point down to beat Tsvetana Pironkova 4-6, 6-3, 6-2 on Wednesday and advance to the U.S. Open semifinals for the 11th time.

Williams, who turns 39 later this month, will meet either Victoria Azarenka of Belarus or Elise Mertens of Belgium in the semifinals.

A winner of 23 Grand Slam tournaments, Williams has lost in four slam finals since giving birth to her daughter on Sept. 1, 2017, as she chases Margaret Court's all-time record of 24. She has won the U.S. Open in New York six times but lost the finals in the past two years.

On Wednesday, she met another mother — Pironkova, a 32-year-old Bulgarian playing in her first match since her son was born in March 2018. In all, nine women with children entered the tournament, and three of them advanced to the quarterfinals.

"It just shows me how tough mothers are," Williams said in an on-court interview after the match. "Whenever you can birth a baby, then honestly, you can do anything. And I think we saw that in Tsvetana today. She played unbelievable."

And she did not make it easy for Williams to move on to another Grand Slam semifinal for the 39th time in her legendary career.

Pironkova capitalized on Williams' lack of energy in the opening set, and her service break in the fifth game put her up 3-2 and in control of the set. She followed by breaking Williams' serve again to open the second set.

But then, Williams found another gear and immediately got the match back on serve by with a break of Pironkova in the second game.

"I was feeling it a little bit in my legs and for whatever reason, an hour in, I get more energy," Williams said of her sluggish start.

She said she was most proud of her determination.

"Keep fighting — that's one thing I'm super excited about is I never give up. Just got to keep going."

In all, Williams broke her opponent's serve four times, and her service game — Williams had 20 aces to four for Pironkova — propelled her to victory. Williams hit 44 winners to 27 for Pironkova.