German star Alexander Zverev could only raise his thumb in salute after his Australian Open semi-final opponent Dominic Thiem delivered an act of perfect sportsmanship on Friday night.
In a tense third set where the match remained up in the air, Thiem forfeited a point that ended up costing him dearly as the two close friends fought for a chance to face Novak Djokovic in Sunday's final.
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In a tight deuce game, Zverev struck a loopy groundstroke that appeared to land beyond the baseline before Thiem half-heartedly returned the ball over the net as the linesman called Zverev's ball out.
Zverev turned to face Thiem immediately after the point only for Thiem to take a close inspection of the mark on the surface and gesture to his rival to challenge the linesman's ruling. According to Channel 9 commentators, he even yelled out across the net for Zverev to send the decision to Hawkeye.
Replays soon showed Zverev's shot had caught the line and he went on to be awarded the point — rather than have it replayed — because Thiem's groundstroke following it sailed long.
Thiem appeared to forget the rules of the game and only put in a half-hearted effort to return Zverev's stroke, despite being aware that the linesman's original call was wrong.
Zverev turned around to give Thiem a thumbs up after the point — but it would have been cold comfort for the Austrian star after he missed his opportunity for a double break in the third set.
Instead Zverev was able to hold at 3-2 — still a break down — before the third set was all tied up at 4-4. Thiem's good karma finally arrived later in the third set when he saved two set points on his serve before taking a two sets to one lead by winning the third set tiebreak 7-3.
Thiem's incredible gesture of sportsmanship stunned Aussie legends Todd Woodbridge and Lleyton Hewitt, who were in the commentary box for Channel 9.
"Thiem is telling him to challenge," Woodbridge said.
"That was good, he is saying. Here comes the challenge. And it is... That is in.
"He probably says, 'Thanks for nothing. I would have won the game'.
"Thiem is going to give him the game."
Woodbridge was just as impressed by Thiem's composure when confronting the chair umpire about the call after the service game, asking if his groundstroke was made at the same time that Zverev's stroke was called out.
When Thiem calmly walked on after a quick exchange with the match official, Woodbridge said: "Thiem's temperament was perfect there.
"He didn't want to get involved and worry about it. Most players would have had a little bit of a discussion. The call came and it affected his shot.
"Thiem handled that incredibly well. He was asked (umpire) John Blom when he sat down on the chair if the shot that he played after that was out. The call was in on his call and he went on with it."
Zverev claimed the first set 6-3 after an exchange of breaks to open the match. Zverev was the first to wake up from the early nerves and got the vital break in the last game by winning the last three points on errors from Thiem.
Thiem saved two break points as he served for the second set before closing with an ace to make it 6-4.
Thiem went ahead 3-6 6-4 7-6 when he came up with brave shots during the big moments in the third set tiebreak.