Katie Boulter progresses to face Aryna Sabalenka in Toronto

British number one Katie Boulter progressed to the third round of the National Bank Open in Toronto after Beatriz Haddad Maia was forced to retire with a back problem.

Brazilian Haddad Maia had come through a marathon first-round match against Marie Bouzkova – which lasted three hours and 20 minutes – but was soon calling for the trainer after the opening game against Boulter.

The world number 33 received lengthy treatment on her lower back and although she tried to resume, Haddad Maia swiftly conceded after Boulter levelled the set at 1-1.

Boulter, who beat Bernarda Pera in the first round, will face Australian Open winner Aryna Sabalenka .

The number two seed kicked off her campaign with a 6-2 6-2 win over China’s Yuan Yue, having missed Wimbledon and the Olympics with a shoulder injury.

Elsewhere on Thursday, top seed Coco Gauff comfortably beat Wang Yafan 6-4 6-4 in her second-round match.