Stan Wawrinka was expected to cruise past Martin Klizan, but the No. 4 seed found his assignment much tougher than he hoped. He was up two-sets-to-one but the Slovakian hit back by taking the fourth to force the match into a decider.

The fifth set was a close affair, but Wawrinka - a three-time grand slam winner - was ultimately too good, closing it out 4-6 6-4 7-5 4-6 6-4.

However, his match was marred by an incident late in the final set.

The former Australian Open champion was booed by the crowd on Rod Laver Arena over a nasty incident that left his opponent Klizan keeled over.

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In the thrilling and see-sawing five-setter, Wawrinka was jeered by the Melbourne crowd after he smacked a forehand smash straight into his opponent's groin at point blank range.

In a tense fifth set, Klizan lobbed a gentle half volley straight to Wawrinka for an easy mid-court put-away and walked aimlessly towards the net without even trying to put himself in a position to return Wawrinka's shot.

Instead, the No. 4 seed cracked his shot straight at Klizan, sparking brief boos from the crowd as the world No. 34 hunched over from the impact of the ball hitting him in his groin.

Wawrinka immediately jumped the net to check on his opponent's condition.

"Goodness me," a Channel 7 commentator said of Wawrinka's hit.

"It's one of those things, Wawrinka would expect Klizan to go left or right and decided to go up the middle not recognising that Klizan had given the point away. He was standing motionless. He's given up on the point and doesn't expect the ball to come his way. That would have hurt."