British number one Emma Raducanu put a dampener on Serena Williams' farewell tour after thrashing the American great in the Cincinnati Open first round.
Williams, 40, plans to retire after the US Open, which starts on 29 August, and lost 6-4 6-0 in what is set to be the penultimate event of her career.
The 23-time Grand Slam champion lacked sharpness and her 19-year-old opponent punished her in a composed display.
Reigning US Open champion Raducanu will play Victoria Azarenka in round two.
"I'm so grateful for the experience to have been able to play Serena and for our careers to have crossed over," said world number 13 Raducanu, who played superbly with her defence of the Grand Slam title in New York looming.
"Everything she has achieved has been so inspirational and it was a true honour to share the court with her."
Meanwhile, British men's number two Dan Evans lost 6-3 6-4 to Serbia's Filip Krajinovic.
Evans, 32, reached the Montreal semi-finals last week but was unable to sustain that form at the second successive ATP Masters event.
Fellow Britons Andy Murray and Cameron Norrie meet in the second round at 16:00 BST on Wednesday, followed on the centre court by Raducanu's match against Belarus' former world number one Azarenka.
Williams puts brave face on chastening defeat
Ever since Williams announced her impending retirement last week - or her evolution away from the sport, as she put it - there has been the swell of acclaim which her legendary status deserves.
The respect of the locker room for Williams has been demonstrated in the following days, with world number one Iga Swiatek and Raducanu among the WTA players lavishing praise on the American's achievements and her legacy.
Four-time major champion Naomi Osaka, who lost in the tournament earlier on Tuesday, cheered on her hero from the stands as Williams took on a player who was not born when she won her first four major titles.
But Williams showed, as many great athletes have in the past, that time has the knack of showing them up on the stages where they once dominated.
There had been reports Williams' preparation was hampered by an injury in the build-up to the match, which was originally advertised to be played on Monday but then moved back to Tuesday.
Whether she was bothered by a physical problem or not, Williams was unable to cope with the pressure caused by Raducanu's serve and groundstrokes.
Now, after waving goodbye to Cincinnati with a smile, the six-time US Open champion must try to regroup for what she will want to be a more fitting farewell at Flushing Meadows.
Raducanu shows signs of US Open form
Raducanu had recalled that her first memory of seeing Williams play was when she went to Wimbledon as a seven-year-old, adding that being drawn against the American on her farewell tour was a "great gift".
But the Briton showed there was no room for nostalgia or charity in one of her best performances since her extraordinary US Open triumph last year.
Williams has long intimidated her opponents by simply being on the other side of the net, with her aura backed up by the potency of her serve and powerful game.
Raducanu showed from the start she was not prepared to be overawed by the occasion.Raducanu hit 14 winners and made just one unforced error in a dominant win which took her just one hour and five minutes
Williams' serve was broken to love in the first game, setting the tone for a one-sided match which slowly deflated the former world number one and, in turn, a home crowd desperate to see a national icon finish her illustrious career with a flourish.
Raducanu broke serve again for a 4-1 lead and, although Williams clawed one of the breaks back to give the crowd hope, the younger player continued to dominate.
A frustrating debut season on the WTA Tour has seen Raducanu struggle to gain momentum but, in what could be her one and only meeting with Williams, she returned to a similar level to the kind of form she showed in New York last year.
After closing out the opening set with a hold to love, Raducanu raced through the second set by dropping just 10 points and sealed a comfortable win with a precise ace down the middle.
"I was nervous from the first point to the last," said Raducanu.
"Serena is dangerous and can come back from any situation. I had to stay focused. I'm so pleased I managed to keep my composure.
"Each of those wins kind of build confidence regardless of the level you're playing, every single win counts. But of course this one more so, because you're playing like the greatest.
"But I'm not trying to get too high or too low right now."