Serena Williams waltzed into the Western and Southern Open quarter-finals on Thursday, while injury curtailed Belinda Bencic and Victoria Azarenka in Cincinnati.

Rogers Cup champion Bencic retired from her third-round clash with Lucie Safarova after dropping the first set 6-2, the Swiss teen citing a right-arm injury.

Bencic later said she did not feel the injury was too serious, but former world number one Azarenka's fitness is more of a concern - the Belarusian quitting midway through her clash with Anastasia Pavlyuchenkova due to a right thigh injury.

It was the first time Azarenka retired mid-match since August 2012, with the world number 21 telling her coach she felt the problem was "getting worse" before indicating she would play no further part.

It was an extraordinary turnaround in the match as Azarenka had raced to a 5-0 lead - when Pavlyuchenkova consulted her coach looking for answers - and sealed a 6-1 first-set lead.

But as Pavlyuchenkova put a ball long on set point, Azarenka - who played with strapping in her win over Caroline Wozniacki - hobbled to the chair, and moved gingerly to start the second set as the Russian raced to a 3-0 lead when the two-time major winner retired.

Williams, meanwhile, won the first nine games of her clash with Karin Knapp before settling for a 6-0 6-2 win.

Knapp was able to win just one point on Williams' serve in the first set, and six in the second - as the American rounded out a routine victory in 54 minutes.

Williams faces Ana Ivanovic next, a player she has an 8-1 head-to-head record against.

Serbian sixth seed Ivanovic fought back to beat Sloane Stephens 2-6 6-4 6-1.

Jelena Jankovic and Anna Schmiedlova set up an unlikely quarter-final clash.

Jankovic, a former world number one, ousted eighth seed Karolina Pliskova 6-2 3-6 7-5, coming from 5-2 down in the third set.

Qualifier Schmiedlova has continued her run at the WTA Premier hard-court event, the Slovakian edging Varvara Lepchenko 6-3 3-6 6-4.

Schmiedlova, who defeated 13th seed Agnieszka Radwanska, has capitalised on Maria Sharapova's withdrawal from the bottom section of the draw.