Australian Open contender Garbine Muguruza has overcome an injury scare to ease into the Sydney International quarter-finals with a straight sets win over Kiki Bertens.

Less than a week after retiring from Brisbane with cramps, Muguruza was leading 2-1 in the first set when she left Ken Rosewall Arena for what appeared to be a back issue.

However the world No.3 showed little ill-effects upon her return to centre court, claiming six of the next eight games to take a commanding lead against the 32nd-ranked Dutchwoman.

Bertens responded with a service break of her own midway through the second and saved a break point in the next game, but eventually fell 6-3 7-6 (8-6).

Muguruza entered the fourth day of the tournament as the only female seed left standing following the second-round exit of Venus Williams on Tuesday.

The reigning Wimbledon champion will now meet the winner of the all-Australian second-round clash between Daria Gavrilova and Samantha Stosur for a spot in the final four.

Earlier on Wednesday, Polish star Agnieszka Radwanksa advanced to her fifth Sydney quarter-final with a straight-sets dismissal of American qualifier Catherine Bellis.

A finalist last year and champion in 2013, Radwanska resisted a strong first-set challenge from Bellis before running away with a 7-6 (7-4) 6-0 win on Wednesday.

“For sure in that match the first set was the key, key point in that match,” Radwanska said.

“It was really tight, and a lot of good tennis in that first set. I think she was a little bit upset about losing that first set.

“I was playing very well in those first few games in the beginning of that second set, and then I was more confident and I was playing more aggressive and using all my chances.”

The former world No.2 will play either two-time Wimbledon champion Petra Kvitova or Italian qualifier Camila Giorgi for a place in the last four.

It’s the fourth time the 28-year-old has reached the quarter-final stage in the harbour city.

“I really like the courts here. I really like to play here. Obviously conditions are tough, sometimes it’s 45. Sometimes it’s so windy. But I think we’re all kind of used to that,” she said.

“But I really like to play here. It’s something that, you know, is in the air that it suits me.”