Williams overcame the varied game of Tsvetana Pironkova 4-6 6-3 6-2 in a thriller in New York to reach the semifinals at her home major for the 11th straight time. Last year's men's finalist, Daniil Medvedev, swept past Russian compatriot Andrey Rublev 7-6 (6) 6-3 7-6 (5) to also make the semifinals.
Williams has erased deficits in her last three matches. She trailed by a set to 2017 champion Sloane Stephens in the third round and was behind 2-0 in the third set to Maria Sakkari -- who beat Williams at a warmup event preceding the US Open -- on Monday.
"I just feel like -- I'm okay," said Williams. "I'm ready to play three sets every match if I have to. It doesn't matter. A win is a win."
Another mom could await in the semifinals Thursday, if twice Grand Slam winner Victoria Azarenka -- who has also had some epic contests with Williams -- downs Belgium's Elise Mertens. The three moms in the quarterfinals were the most ever at a major in the Open Era.
Williams had to rally from a set and break down against Pironkova, who officially has no ranking after playing her first tournament in three years.
Helping her decision was a rule change allowing moms to come back with the same ranking they held when they left, for a certain number of events.
Still, Pironkova's special ranking sits at 123rd, which in normal circumstances would have meant only a spot in qualifying, not the main draw.
But with qualifying scrapped to limit the numbers on site at Flushing Meadows as part of the US Open's bubble and other players withdrawing -- mostly due to coronavirus concerns -- the 32-year-old snuck into the 128-player singles field. There are no fans at this year's tournament, too.
She eliminated a two-time Grand Slam champion, Garbine Muguruza, in the second round and came close to ousting the 38-year-old Williams in what would have been one of the biggest Grand Slam upsets in recent history.
"So it's quite a long journey, and it's been amazing. It still is," said Pironkova. "I just feel privileged to be here and to be able to play my favorite sport in such a big arena. So it's amazing."
The contest featured several highlight reel rallies, including one of 24 shots in a game that Williams broke in for 5-3 in the second set. Pironkova dragged Wiliams from side to side in the early going, with her opponent doing more of it herself later. The Bulgarian also employed her unorthodox slice forehand.
They traded laser backhands down the line and such was Pironkova's disguise on her serve that Williams was forced to use her left hand on a return. It led to a break of serve.
"Actually been hitting a little lefty in my practice but not on purpose," Williams said. "I noticed the other day, I hit a lefty shot and I was just in practice being super intense, and I guess it came through in the match.
"So when you're in a position like that...this is the quarterfinals of a Grand Slam, and I was just trying to do everything I can whether righty or lefty."
The first game of the decider proved pivotal.
With Pironkova holding a game point, she was unable to put away a forehand volley, allowing Williams to rip a pass to finish the point. Williams made her pay by breaking and never trailed again.
Willliams got an insurance break for 5-2 after Pironkova built a 40-15 advantage. She hit 20 aces overall, her most in eight years. The two exchanged a smile at the net when it ended.