The best 21-and-under players in the world took part in a fun serving challenge at this year's ATP Finals in Milan.

Frances Tiafoe, Jannik Sinner, Casper Ruud, Mikael Ymer, Ugo Humbert and Alejandro Davidovich Fokina all tried to hit a down-the-tee service target. The key was to knock down the target in the least amount of attempts.

Tiafoe came out pretty confident, posting a low score of seven attempts. Howeve,r Ymer closed the deal for Tiafoe when he put up an even lower score of six. But it was Sinner who had the final say. The ATP Finals champion crushed the competition when he needed just four serves.

Sinner's serve helped him win in Milan, and then seal his last title of the year at an ATP Challenger event in Italy.