The 40-year-old fell to 0-2 in matches since announcing "the countdown has begun" on her career, losing 6-4 6-0 to Emma Raducanu at the Cincinnati Open.

Williams said last week that her career was winding down, although she did not explicitly say the US Open, which begins August 29 in New York, would be her last tournament.

The Cincinnati event was the second US Open tune-up for Williams, and the next time she takes the court will be at Flushing Meadows. She lost to Belinda Bencic in straight sets last week in Toronto. A day before the announcement, Williams won her first match since the 2021 French Open.

Williams is a 23-time grand slam champion, most recently in 2017 at the Australian Open, when she was pregnant with daughter Olympia. She said wanting to expand her family was a big reason she plans to step away.

Raducanu, the 10th-seeded British teenager, was sharp as she dispatched Williams in just 65 minutes.

"I can't believe I just played Serena Williams," the 19-year-old world No.19 said.

"It's something that I think I'm really fortunate to have been able to do, and for our careers to have crossed when there's such a big (age) gap and watching her growing up, it was an amazing experience to just play her.

"I think that the crowd was pretty electric. The stadium was really packed, and even if they were cheering for Serena ... I was prepared for that."

Williams did not speak to reporters after the match.

Raducanu, who faces veteran Victoria Azarenka in Wednesday's second round, has not won a title since her out-of-nowhere triumph at the US Open last year.

Elsewhere, current world No.2 Anett Kontaveit of Estonia came from a set down to defeat Czech Tereza Martincova 3-6 7-5 6-4 but world No.3 Maria Sakkari of Greece was beaten in three sets, 6-7 (2-7) 7-6 (8-6) 1-6 by France's Caroline Garcia.

Four-time grand slam champion Naomi Osaka was eliminated by China's Zhang Shuai 6-4 7-5 while another former world No.1 in Simona Halep, the 15th seed, was a 6-4 3-6 6-3 winner against Russia's Anastasia Potapova.

Karolina Pliskova, the No.14 seed and 2016 Cincinnati champion, eased into the round of 32 with a 7-5 6-1 victory over Venus Williams.

Belgium's Elise Mertens brushed aside Ukraine's Anhelina Kalinina 6-1 6-1 to set up a second-round date with Pliskova and Romania's Sorana Cirstea knocked out 12th seed Belinda Bencic 6-2 6-7 (3-7) 6-4.

With AP.

Australian Associated Press