Garbine Muguruza's hopes of making a winning start to 2016 were ended when the second seed retired from her Brisbane International second-round match against Varvara Lepchenko with a foot injury.

The world number three, who was given a bye in the first round, called for treatment after losing the opening set 7-6 (11-9) on Wednesday and decided she could not continue after the American won the opening game of the second set.

"I thought I could play with it, but it just got to the point where I had to stop." said last year's Wimbledon runner-up.

The Spaniard, who had looked out of sorts as she made 27 unforced errors in the first set, was among the favourites to win the tournament after Maria Sharapova and Simona Halep also withdrew hurt.

Belinda Bencic may have fancied her chances of being crowned champion this week, but the seventh seed is out after Samantha Crawford upset the odds to win in straight sets.

American qualifier Crawford served 13 aces to secure her first victory over top-50-ranked opponent, which finally came when she won her six match-point to put Bencic out of her misery.

Fourth seed Angelique Kerber reached the quarter-finals by brushing aside Madison Brengle 6-3 6-0 in just over an hour.

Victoria Azarenka was a 6-3 6-2 winner against lucky loser Ysaline Bonaventure, while Andrea Petkovic got the better of Ekaterina Makarova in straight sets.