Serena Williams, of the United States, reacts during a match against Tsvetana Pironkova, of Bulgaria, during the quarterfinals of the US Open tennis championships, Wednesday in New York. (AP Photo/Seth Wenig)

NEW YORK (AP) — They were just two particular points from Serena Williams’ latest three-set comeback at the U.S. Open, yet they were pivotal and consisted of the sorts of lengthy exchanges filled with athleticism and brilliance that in any other, non-pandemic year would be marked by thousands of folks rising to their feet for delirious roars and raucous applause.

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.

“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.”

How big an upset would this have been if Pironkova had held on? Not only is she not seeded at Flushing Meadows, she doesn’t even appear in the WTA rankings at all — this was her first tournament of any sort in more than three years, because she left the tour to become a mother.

“It’s unbelievable,” Williams said about Pironkova’s impressive return to competition. “Wow. I couldn’t even do that.”

When the players stepped out onto the court, the stadium announcer — announcing for whom, exactly, was something of a mystery — referred to Pironkova, a 32-year-old from Bulgaria, as “Alexander’s mom” and then to Williams as “Olympia’s mom” during the pre-match introductions.

“It just shows me how tough moms are,” Williams said afterward.

“You play a match and you go home and you’re still changing diapers,” said Williams, whose daughter turned 3 on Sept. 1 and is a little older than Pironkova’s son. “It’s like a double life.”