TORONTO: Karolina Pliskova kept the pressure on Naomi Osaka in the battle to take the world number one ranking by reaching the quarter-finals of the WTA tournament in Toronto on Thursday.

The Czech third seed saw off Estonia's Anett Kontaveit 6-3, 7-5 to set up a last-eight clash with either Canadian rising star Bianca Andreescu or fifth-seeded Dutchwoman Kiki Bertens.

Pliskova, 27, must reach the semi-finals to have a chance of reclaiming the world number one ranking but if Osaka makes the last-eight, then Pliskova would need to reach the final to stay in contention.

French Open champion Ashleigh Barty's stint as the world's top-ranked player ended after she was knocked out on Tuesday.

Japanese star Osaka, the reigning US and Australian Open champion, takes on Polish teenager Iga Swiatek in her last-16 tie later on Thursday.