Struggling Swiss former winner Stan Wawrinka was knocked out of the Australian Open by American Tennys Sandgren in the second round on Thursday.

The 97th-ranked Sandgren defeated ninth seeded Wawrinka 6-2, 6-1, 6-4 and will next play Germany's Maximilian Marterer.

It has been tough so far this year for Wawrinka, who was playing in his first tournament since Wimbledon six months ago following left knee surgery.

He was never in the contest and had his serve broken five times and made only 21 winners and 35 unforced errors.

The three-time Grand Slam winner, who defeated Rafael Nadal to win the 2014 Australian Open, has slipped to nine in the world rankings and faces a battle to climb higher after his early round exit.

He made the semi-finals at last year's Australian Open where he lost to eventual champion Roger Federer in five sets.

Wawrinka had a troubled lead-in to the year's opening Grand Slam, pulling out of an exhibition event in Abu Dhabi on his way to Australia.

He has not played a competitive match prior to his first round Open win over Lithuania's Ricardas Berankis, having only decided to take part last weekend.

Sandgren's next opponent, world No.94 Marterer, reached the third round with a 6-4, 4-6, 7-6(5), 3-6, 6-3 over Spanish veteran Fernando Verdasco.