Venus Williams ousted defending champion Kiki Bertens at the Western and Southern Open in Cincinnati.
The seven-time grand slam champion beat the number five seed 6-3 3-6 7-6 (4) in a titanic second-round tussle that lasted two hours and 17 minutes.
Bertens had beaten Williams in a three-set thriller at Wimbledon last month.
But the 39-year-old American held her nerve this time to claim her second top-five victory of the season, alongside her win over then-number three Petra Kvitova at Indian Wells.
Meanwhile, Serena Williams withdrew from the tournament citing a back problem which forced her to pull out of the Rogers Cup final at the weekend.
The American was due to face Zarina Diyas in the first round in Ohio on Tuesday evening.
Quoted on the WTA site, she said: "I am so sad to withdraw from the Western & Southern Open as it is truly one of the tournaments I most love to play."
Elsewhere, unseeded Anett Kontaveit upset former world number one Angelique Kerber 7-6 (7) 6-2 in their first-round match, while Madison Keys squeaked past 2017 champion Garbine Muguruza 6-7 (4) 7-6 (5) 6-4.
There were also wins for Victoria Azarenka, Donna Vekic, Jennifer Brady, Ekaterina Alexandrova and Yafan Wang.