BRISBANE - Former world number one Victoria Azarenka moved into the quarter-finals of the Brisbane International when she downed little-known Belgian Ysaline Bonaventure 6-3, 6-2 on Wednesday.

Bonaventure, who had lost in the final qualifying round, gained a main draw spot when top seed Simona Halep pulled out with a leg injury on Tuesday. She pushed the two-time grand slam winner hard, particularly in the 56-minute first set when she blasted 18 clean winners.

But once Azarenka weathered the storm she was able to master the 21-year-old Belgian and was never really troubled thereafter. "I think the beginning definitely was really tough," said 26-year-old Azarenka. "She played very well. You know, it's always a little bit of a moment of adaptation when you don't know who you play, so it's an adjustment.

"But I'm glad I could stay strong in the beginning and turn it around, because it wasn't going my way at first for sure."

Azarenka spent 51 weeks as world number one in 2012 but suffered a severe foot injury in 2014, finishing the year at 32. She was then hampered by a left thigh problem in 2015, which affected her climb back to the top 10. Azarenka started the year at number 22 and is unseeded this week. She said she was pleased with how she had been able to take control of the match midway through the first set.

"I think just to feel the momentum shift in the match was important, you know, in the beginning of the year," she said. "Especially since I haven't played in a while. So it's good to go through those moments." Azarenka will play 2015 US Open runner-up Roberta Vinci in the quarter-finals. "She's been playing really well and showing some great results," Azarenka said of the eighth-seeded Italian. "I think you can see improvement in her game. I think she's just on the rise and she feels really good, confident."

Fourth-seeded German Angelique Kerber was impressive in her 6-3, 6-0 demolition of American Madison Brengle. The world number 10 will now take on Russia's Anastasia Pavlyuchenkova in the quarter-finals.

Little-known American Samantha Crawford upset seventh-seeded Swiss Belinda Bencic 7-5, 7-5 in 90 minutes. The qualifier from Florida will now take on Germany's Andrea Petkovic, who ousted Russia's Ekaterina Makarova 7-5, 6-4.

Petkovic conceded she knew nothing of the 20-year-old Crawford. "I don't know anything -- it's going to be classic YouTube stalking from my side," she said. "I don't know if I'm going to find a lot, but she must be playing tremendously well to beat Bencic and win three qualifying rounds."

Nishikori untroubled: Japan's Kei Nishikori overcame a slow start to cruise past Mikhail Kukushkin and move into the quarter-finals of the Brisbane International on Wednesday.

Nishikori lost his opening serve to go 2-0 down in the first set but soon recovered to get on top of Kukushkin and ease to a 6-3, 6-4 win. "I thought it was a great match," the second seed said. "(But) I didn't start quite well -- I think he really started well."

Nishikori, who reached the final of the US Open in 2014, was below his best in his first match of the season, particularly during the opening set. His serve was initially poor but it improved as the match progressed and he faced only one break point in the second set.

"I think I served really well today. That's also improving, so I'm very happy for that," he said. Nishikori also found his range with his groundstrokes and was able to force errors from his opponent from Kazakhstan.

"He was playing aggressive, great backhand, and I tried it mix up," Nishikori said. "I think I went to his backhand too much (but) I tried to move the ball around and start playing better."

It was Nishikori's fourth straight win over Kukushkin and ensured he reached at least the quarter-final stage in Brisbane for the fourth time in five years. The world number eight will now play seventh-seeded Australian Bernard Tomic, who beat Czech veteran Radek Stepanek 7-6 (8/6), 4-6, 7-6 (7/4).

Big-serving Croatian Marin Cilic, who beat Nishikori to win the 2014 US Open, downed rising Korean star Chung Hyeon in two tight sets 7-5, 7-6 (7/3). Cilic will take on Austrian Dominic Thiem after the eighth seed's comfortable 6-3, 6-2 win over Denis Kudla of the US.