Injured Serena pulls out of Cincinnati

Serena Williams has withdrawn from the Cincinnati Masters, citing a back problem which forced her to pull out of the Rogers Cup final at the weekend.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 and her latest setback places her in some doubt for the US Open.

"I am so sad to withdraw ... I came to Mason on Sunday and have tried everything to be ready to play tonight, and was still hopeful after my practice this morning," she said on the WTA site.

"But unfortunately my back is still not right and I know I should not take to the court."

The 23-time grand slam winner retired while trailing 3-1 against Bianca Andreescu in Toronto as a Canadian woman took the Rogers Cup for the first time in 50 years.

Meanwhile Estonian Anett Kontaveit ousted German 13th seed Angelique Kerber 7-6 (9-7) 6-2, also on Tuesday.

The 23-year-old Kontaveit came from 5-3 down and saved a set point at 6-5 on the way to taking the opening set against the former No.1.

She broke Kerber twice to win the second set, winning the first nine points, and two holds to love gave her a 4-0 lead.

Swiss 12th seed Belinda Bencic retired with a foot injury after trailing former world No.1 Victoria Azarenka of Belarus 6-4 1-0.

Russia's Ekaterina Alexandrova, Zheng Saisai of China and Croatia's Donna Vekic were among other early winners.

Alexandrova turned back Carla Suarez Navarro of Spain 6-4 6-1, Zheng beat Viktoria Kuzmova of Slovakia 6-3 6-1 and Vekic, who will face Azarenka in the second round, was a 6-4 6-3 winner over Czech Katerina Siniakova.

American world No.16 Madison Keys defeated Spaniard Garbine Muguruza 6-7 (7-4) 7-6 (7-5) 6-4 after blasting 11 aces and 56 winners.

This saw Keys earn revenge on Muguruza, who knocked her out of Cincinnati in the third round the last time the pair met at the tournament in 2017 despite holding three match points.

"I totally forgot I had match points, so thanks for reminding me," she joked after the match.

"I was just really happy to be able to fight back in that second set and get back into the match.

"It definitely gives me confidence. I think I not only mentally picked myself up but I started playing better in the second and third sets."

Former world No.1 Venus Williams beat defending champion Kiki Bertens 6-3 3-6 7-6 (7-4) in a two-hour arm wrestle.

Williams will play the winner of Vekic and Azarenka in the third round.