'Star striking' - Swiatek inspired by chat with Serena Williams
Current world number one Swiatek was spotted talking with Williams in the players' gym at Flushing Meadows.
Far from finding it a disruption to her match preparations, Swiatek said it "more inspired me and it gave me, like, a positive kick, so it was nice."
Williams, wearing a throwback look of a short pleated denim skirt and jacket, returned to the venue where she lifted the trophy six times, sharing the record for most US Open women's singles titles with Chris Evert.
She last played the tournament in 2022, when she had already said she planned to "evolve away" from tennis once that year's final Grand Slam was over.
"It was really nice to see her," Swiatek said. "She has a lot of positive energy. It's nice that she came on-site and she was chatting with the players.
"Even though we met before and for like a couple of years we have been on the same sides and on tour together, she's still like star-striking me," Swiatek said.
"It was nice that she approached me, because I wouldn't, for sure, find courage to do that. She's really nice and really positive."
Williams, who remains busy with various business enterprises and welcomed the birth of her second daughter last year, was spotted in the stands at Arthur Ashe Stadium watching men's world number one Jannik Sinner's win over Christopher O'Connell.
She also caught some of American Tommy Paul's win over Gabriel Diallo.
Her old friend Caroline Wozniacki, back on tour after a maternity break, joked that she was "pretty mad" that she didn't spot Williams in the stands at her match.
"I'm happy that she's following tennis and my game because she told me that she's cheering for me," Swiatek said. "It's always nice to hear that from somebody like Serena."
Swiatek, the 2022 US Open champion, certainly looked sharp in her 6-4, 6-2 victory over 27th-ranked Pavlyuchenkova, a potentially tricky opponent with a French Open runner-up finish and seven other Grand Slam quarter-finals on her resume.
Swiatek will meet 16th-seeded Liudmila Samsonova of Russia for a place in the quarter-finals.