The 23-time Grand Slam winner and former first-ranked Serena Williams lost to Emma Raducanu in the first round of the tournament in Cincinnati. The British player won 6-4, 6-0. The match lasted one hour and five minutes.
The 40-year-old American said she would quit her career after the US Open.
The prize fund of the tournament in Cincinnati (WTA Western & Southern Open) is $2.5 million.