Switzerland's Stanislas Wawrinka upset injury-troubled world number one Rafael Nadal to win his first Grand Slam title at the Australian Open on Sunday.

The eighth seed roared through the first two sets and overcame the distraction of a back injury to Nadal, and then the loss of the third set, to take it 6-3, 6-2, 3-6, 6-3.

The top-seeded Spaniard seemed to strain his back early in the second set and left the court for a medical time-out but after surrendering the set, he returned to the court swinging to stun Wawrinka with an early break.

With his shot-making showing signs of a revival, Nadal saved two break points before holding serve with a barrage of sweetly-struck forehands.

A searing forehand winner down the line gave Nadal two break points and astonishingly, the Spaniard broke a flat-footed Wawrinka when the Swiss bunted a bloodless forehand into the net.

As Nadal rallied, his eighth-seeded opponent sunk, spraying shots all over the court with his concentration clearly affected by the sudden momentum switch.

Still restricted in his movements, Nadal began gunning for the lines and his stand-and-deliver approach saw him march to a 5-2 lead.

With Nadal serving for the set at 5-3, Wawrinka had a chance to break back but was unable to flush the tension out and conceded the set when he smashed a forehand into the net.

The Swiss won the first set, broke Nadal in the opening game of the second and held serve to take a 2-0 lead before Nadal grimaced in pain and clutched at his back after playing a shot from the baseline midway through the next game.

Nadal successfully held serve to trail 2-1 but quickly left the court for a medical time-out, leaving Wawrinka to argue with the chair umpire about the lack of disclosure over the injury.

CROWD JEERS

Nadal re-emerged after six minutes amid some jeers from the Rod Laver Arena crowd but his movement was clearly restricted and he was unable to generate any pace on his serve.

Wawrinka simply went about his business, marching to a 4-1 lead as Nadal double-faulted repeatedly and struggled to reach wide balls.

Nadal had more treatment on his back from a physio at the change of ends, then underlined his fighting spirit by holding serve to trail 5-2, but Wawrinka went on to serve out the set with an ace.

In the first set, Wawrinka captured an early break to surge 3-1 clear and saved three break points before serving out the set, the first time he had taken one off Nadal in 13 matches.

With both players serving strongly in a smouldering start, the match suddenly came alight in the fourth game when Nadal made his first double-fault and Wawrinka crunched a passing shot to bring up two break points.

In a tense rally, the Swiss ratcheted up the pressure, with a big crosscourt forehand that Nadal was only able to parry into the net.

Without a hint of nerves, Wawinka blasted two aces before holding to lead 4-1, leaving the Rod Laver Arena crowd stunned.

The nuggety 28-year-old then tested the Spaniard again, punching a commanding backhand volley to raise another break point, but the Spaniard saved it.

Serving for the set at 5-3, Wawrinka stumbled, unable to land a first serve for five consecutive points, to concede three break points.

He saved them all as Nadal's returning went awry and he took the set with a cross-court ace.