World number two Garbine Muguruza suffered a fitness scare ahead of this month's Australian Open when she was forced to retire from the Brisbane International on Tuesday with severe cramping.

The men's draw also suffered a major blow when Andy Murray withdrew -- the second star player to pull out after top seed Rafael Nadal did so late last week.

Wimbledon champion Muguruza was ahead 2-1 in the deciding set against Serbia's Aleksandra Krunic in the second round of the warm-up tournament for the season's first Grand Slam event, when she collapsed after a serve.

The Spanish top seed could not continue and handed the match to Krunic 5-7, 7-6 (7/3), 2-1.

"I felt in trouble in the second set when I was 2-0 up," Muguruza said. "I started to feel my calves were cramping."

Muguruza had won a tight first set and appeared heading for a straight sets win over Krunic when she opened up a 5-2 lead in the second. However, she began to struggle in the 30 Celsius heat and oppressive humidity as Krunic fought back to win the second set on a tiebreak. Muguruza received treatment from the physio on court before the start of the decider and broke Krunic, only to collapse while serving to consolidate the service break.

Murray, who has not played on the ATP tour since losing a five-setter to American Sam Querrey in the quarterfinals at Wimbledon, issued a statement Tuesday announcing his withdrawal from the tournament, and did not say whether he would be able to play the year's first Grand Slam.