Jack Sock was the butt of all jokes after his loss to Roger Federer after managing to unsettle the Swiss legend with his backside.
Federer started his bid for ATP Finals glory with a commanding 6-4 7-6 (7-4) victory over Sock on Sunday.
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The Swiss, a six-time winner of the event, was always in control of the Group Boris Becker clash against the tournament's surprise package, although Sock managed to make Federer look rather silly at point during the first set.
After Sock hit an awkward volley that left Federer with an easy winner, the young American turned his back and stuck out his backside at his opponent.
Whether or not he was distracted by the strange move remains to be seen, but Federer ended up putting the easy forehand straight into the net.
Sock couldn't believe his 'cheeky' tactic worked, and after the match Federer said he should have just aimed for the 'big' target Sock was giving him.
"It was a big distraction," he said. "Because it was very big."
"That's what I should have aimed for.
"That target was bigger than the down the line corridor that I had."
Sock, who made the cut after winning the Paris Masters, did manage to shake off his early nerves to produce a commendable showing, particularly in the second set, but Federer was ultimately too strong.
With Federer in his groove, the scale of the task facing Sock was evident and the American did at times look overawed by the occasion.
Sock started the second set in style, holding to love in a game sealed by a stunning cross-court forehand that left Federer stranded.
The 25-year-old underdog provided prolonged resistance and dug deep to fend off five break points across three service games to lead 6-5, putting pressure on his illustrious rival.
Federer coolly took it to a tie-break, where he eased over the line to secure a deserved win.
Federer will play Alexander Zverev and Marin Cilic in his remaining pool matches, with the top two progressing to the semi-finals.
Zverev edged Cilic 6-4 3-6 6-4 in Sunday's night match with Rafael Nadal to open his campaign on Monday against Belgian David Goffin.
Dominic Thiem and Grigor Dimitrov will clash in Monday's other match.