Serena Williams kept her bid for a record-equalling 24th Grand Slam title alive as she came from a set down to beat Tsvetana Pironkova and reach the US Open semi-finals.

The six-time champion fired down 20 aces as she won 4-6 6-3 6-2 - her third successive three-set win - in two hours and 14 minutes.

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She will face either Victoria Azarenka or Elise Mertens in her 14th US Open semi-final.

"I keep fighting,” said Williams. That’s one thing I am super excited about, I never give up and I just have to keep going.

It’s not over. I am just happy to still be out here and I want to play better.

Williams was also full of praise for fellow mother Pironkova, who was playing in her first tournament in three years after taking time away from the game.

"If you can birth a baby, you can do anything,” said Williams. "We saw that with Tsvetana today. She played unbelievable. I could barely win a match when I came back. She’s incredible.”

Tsvetana Pironkova during her US Open quarter-final against Serena Williams

Image credit: Getty Images

Pironkova started brightly and troubled Williams in the opening set with her mix of slices and powerful forehands.

She got the only break of the first set in the fifth game and then broke again in the opening game of the second set. But Williams hit back immediately and started to raise her level.

With her serve firing on all cylinders, she applied more pressure on Pironkova, who wasn't able to maintain her consistency as the match wore on.

Williams broke late in the second set to level and then again early in the third before powering her way to victory.

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