Williams also experienced a three-set contest in her last-16 match, but her stamina appeared to increase as this match went on.
"It's not how you start, it's how you finish, right?"
Williams' last Slam title came at the 2017 Aussie Open - about seven months before the birth of her first child.
"I never give up and you know I just gotta keep going", Williams added.
"I think her feelings about Black Lives Matter and police brutality in the United States, and obviously the racism that she's experienced, those feelings have been brewing for a while".
How big an upset would this have been if Pironkova had held on?
For the first time in Grand Slam history, three moms are in the quarterfinal: Williams, Victoria Azarenka, and Tsvetana Pironkova. "Yeah, yeah, that was intense", Williams said. "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.
"I've just got to be a little bit better, really, and just expect people to go lights out and just be ready for lights out", Williams said.
"You play a match and you go home and you're still changing diapers", said Williams, whose daughter turned 3 on September 1 and is a little older than Pironkova's son. Her more decorated opponent raised her arm and let out a cry of celebration and relief inside the near empty Arthur Ashe Stadium.
Serena Williams is preparing herself for "lights out" tennis from Victoria Azarenka as the two mothers face off for a place in the US Open final.
However, Azarenka has the ability to make Williams move all over the court and test her for a prolonged period of time.
Pironkova, who beat Venus Williams en route to the Wimbledon semi-finals in 2010, had seized advantage of a new rule which allows players who have taken time off the tour to give birth to return at their original ranking.
The other women's semifinal Thursday will be 2018 champion Naomi Osaka vs.
In the second, Williams won a 24-point rally to break and open up a 5-3 lead and then, after falling behind 15-30 on her serve, fired three consecutive aces to force a decider.
"But I say that never wanting to write-off Serena, because if anyone can produce a rabbit out of the hat in the next few days, it's Serena". No. 2 seed Dominic Thiem has reached three, including this year's Australian Open, while No. 3 Daniil Medvedev has played in one - at last year's U.S. Open.
Pironkova agreed that it was hard juggling her dual role.
The other came in a four-deuce opening game of the final set, and began with the right-handed Williams taking a page out of old friend Maria Sharapova's playbook by hitting a left-handed return of serve. "I'm just so excited for this opportunity and I hope people are excited for this match".
Pironkova dropped onto her back, chest heaving; she left so much sweat on the court that a ball person was beckoned to wipe it away with a towel.
Williams had a break point in the fourth game, as Pironkova's first serve began to falter.
She said she was benefitting from staying in a private house in NY, rather than one of the tournament's pre-approved player hotels.
However, Williams didn't have to go into her Women's Singles third round match against Sloane Stephens without any support. "She's a really good player".
Yet Pironkova's story heading into their last-eight meeting was perhaps even more remarkable.
"I tried everything", Pironkova said.
Williams, in contrast, made 11 in the first set, just 13 the rest of the way.
"Definitely, the first set, I think I was in control of the match".