Kvitová, seeded eighth, overcame the 23-time Grand Slam champion 6-3, 2-6, 6-3 at the Lindner Family Tennis Center.

The defeat for Williams, who withdrew from the recent Rogers Cup in Canada after admitting she was struggling with post-natal emotions, ended her pursuit of a third title in Cincinnati.

The 28-year-old Czech will take on French player Kristina Mladenovic, who comfortably dispatched Viktória Kužmová of Slovakia 6-3, 6-0, for a place in the quarter-finals.

"It was a pleasure to share the court with Serena, she's such a champion," said Kvitová.

"We are both coming back - from injury, from motherhood - so it's something really special to play her."

Fifth seed Elina Svitolina of Ukraine joined two-time Wimbledon champion Kvitová in the next round after she defeated Russia's Svetlana Kuznetsova 7-6, 4-6, 6-4.

Caroline Garcia of France, the sixth seed, is also through following a 6-4, 7-5 win over Belarus' Victoria Azarenka.

In the men's draw, top seed Roger Federer cruised into round three with a straightforward 6-4, 6-4 victory against Peter Gojowczyk of Germany.

The 20-time Grand Slam winner, making his first appearance at the event since he claimed the title in 2015, became the top-seeded player following the withdrawal of Rafael Nadal.

Federer will play Argentina's Leonardo Mayer, who recorded an impressive 7-6, 6-4 triumph over Frenchman Lucas Pouille, in the next round.

Canadian Denis Shapovalov also reached the third round as he ousted Britain's Kyle Edmund 6-4, 7-5.

The event is due to continue tomorrow.