Wawrinka two sets up against struggling Nadal

Melbourne (AFP) - Switzerland's Stanislas Wawrinka took a shock two-set lead against Rafael Nadal as the world number one appeared affected by a back injury in the Australian Open final on Sunday.

Nadal lost the first set 6-3 to an inspired Wawrinka and he winced and clutched his back during the third game of the second set, and went off the court for a medical timeout at 2-1 down.

When he returned, he was clearly impeded and had further treatment on court at 4-1 down, as well as after losing the second set 6-2.

Nadal entered as the hot favourite in his 19th Grand Slam final against eighth seed Wawrinka, 28, who is contesting his first major title match.

The Spaniard has previously been plagued by injuries at the season's first Grand Slam. He missed the 2006 and 2013 editions, and had to retire injured in 2010 while playing Andy Murray.

Nadal was also hit by a leg muscle strain in his 2011 quarter-final loss at Melbourne Park to David Ferrer.