The Australian qualifier upset world No.9 Williams 6-3 2-6 6-2 to claim the biggest scalp of her career and advance to the third round of the key US Open lead-up event.
It was Barty’s first success against a top-10 player and follows the 21-year-old’s win over Wimbledon champion and former world No.2 Garbine Muguruza in the Birmingham semi-finals in June.
“It feels really good. To play Venus on such a big stage is a privilege for me and to play as well as I did, I’m really excited,” Barty said.
“She’s a genuine champion of the game and paved the way for us young players coming through.
“It’s an honour for me to… share the court with her, and happy to be moving on.”
Barty will face former world No.1 Caroline Wozniacki for a place in the quarter-finals after the sixth-seeded Dane beat Russia’s Elena Vesnina 6-2 6-4.
Ninth-seeded Williams, the runner-up at the Australian Open and Wimbledon this year, fell behind Barty 4-1 in the decisive set after dropping serve in the third and fifth games.
Barty showed plenty of poise to serve out the match to love to finish off Williams in one hour and 50 minutes.
Williams was full of praise for the former Wimbledon junior champion, who has already landed her maiden WTA title this season in Malaysia.
“She just played really well. She found openings in the court, was super consistent, and she just played really well. I just have to give her credit,” Williams said.
Stosur had held Australia’s top women’s ranking for a record 442 weeks, but will find herself as the country’s No.3 when she arrives in New York for the final grand slam of the year starting on August 29.
Barty is set to rise to a career-high ranking of at least No.42 in the world following her win over Williams, with Stosur languishing at 48th having not played since suffering a stress fracture in her right hand at the French Open in early June.
While Barty marches on, Australian No.1 Daria Gavrilova was eliminated in Cincinnati by world No.82 Camila Giorgi 7-6 5-7 6-3.
American 16th seed Madison Keys beat Russian Daria Kasatkina to reach the last 16, where she will face fourth-seeded Muguruza.
Eighth-seeded Russian Svetlana Kuznetsova beat Kazakhstan’s Yulia Putintseva to set up a showdown against Spain’s Carla Suarez Navarro.
Germany’s Julia Goerges and Slovakian 11th seed Dominika Cibulkova also had wins.
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