British number one Emma Raducanu put a damper 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.