Emma Raducanu defeated Serena Williams 6-4 6-0 in their first and likely last encounter on the court.
Williams, 40, struggled to find consistency in just her fourth match of the season, giving up 12 unforced errors while making just 11 winners in the first round of the Western and Southern Open in Cincinnati.
Meanwhile, the 10th-seeded Brit settled in to her rhythm early, breaking the 23-time major champion twice in the opening set before racing away in the second to claim her 12th win of the season.
The 19-year-old paid tribute to Williams in her on-court interview after the match.
“I think we all need to just honour Serena and her amazing career,” she said.
“I’m so grateful for the experience of being able to play her and for our careers to cross over.
“Everything that she’s achieved is so inspirational and it was a true honour to share the court with her.”
Raducanu hit just one unforced error in her victory over Williams and intends to take all the momentum she can into her next match against another WTA Tour veteran in Victoria Azarenka.
“I think it’s important to really cherish the wins, because I think that if anything, this year has taught me that the wins don’t come by easily,” said Raducanu, who has struggled with blisters on her hands.
“No matter what win that is, it means a lot to any athlete, and I definitely am going to take it in, be really proud of myself.”
Raducanu added: “It is a fast turnover tomorrow, and I probably have got 12 hours until I’m back on site. I’m playing another slam champion, so it’s obviously going to be really difficult. Just going to try to prepare as best as possible.”
Belarusian former world number one Azarenka overcame Kaia Kanepi in the opening round.
Looking ahead to the match, Raducanu said: “I remember more recently when she was playing in the US Open and made the final in 2020 during the bubble, I was studying for my exams.
“I hadn’t played tennis in like two, three months, and I was kind of just dreaming that one day I’m going to play on Ashe (Arthur Ashe Stadium at Flushing Meadows), and one day I’m going to be at that level and playing these great players myself. Everything felt so far away.”