Rafael Nadal.

THE ASSOCIATED PRESS

MADRID, Spain -- Rafael Nadal will undergo further medical scans in Spain to determine the severity of the knee injury that forced him out of the Australian Open.

The second-ranked Nadal retired during his quarter-final against Andy Murray after losing the first two sets and trailing in the third, complaining of discomfort in his right knee. Tendinitis in that same knee forced him to miss part of last season, including Wimbledon.

"We don't know the scope of the injury but there is something going on in there, something that wasn't there before," uncle and coach Toni Nadal told Spanish media from Australia. "Something felt wrong and it was the right decision to retire."

Nadal is set to undergo MRI scans after arriving in Spain to determine the extent of the problem, with Toni Nadal quoted as saying that a preliminary ultrasound showed it "wasn't a serious problem."

"You're always worried about these things, it's normal," Toni Nadal said. "It appears not to be what he had before and something else, but we must be prudent and wait and see."