Roger Federer is the most decorated tennis champion of all time. He's won 17 Grand Slam titles — seven at Wimbledon, five U.S. Opens, four Australian Opens and one French Open.

Yet Federer was shown up in front of the world by a young fan during an exhibition against Grigor Dimitrov at Madison Square Garden on Tuesday.

Tied at one set apiece and with Federer leading 3-2 in the third and up 15-0 while serving, a boy came down and challenged Federer to a point. 

Federer took it easy on the kid and perhaps he shouldn't have. After getting into a rally, Federer came to the net and volleyed a shot back at the kid. Then the young boy executed a perfect lob over the world champion's head for a winner. 

Federer's face said it all.