A tennis fan tried to steal something from Jannik Sinner's bag after his Round of 16 win over Alexander Bublik. / Screengrab on Twitter/ @TheTennisLetter

World No. 1 Jannik Sinner made quick work of his Round of 16 opponent, Alexander Bublik, to book a spot in the U.S. Open quarterfinal Monday night. The Italian was likely ready to call it a night when he became victim to a tennis fan standing courtside who tried to steal something out of his bag.

After Sinner's dominant straight-sets victory over Bublik at Arthur Ashe stadium, he walked over to give his towel to a fan, which was considered a pretty typical post-match interaction. While his back was turned, however, another fan was caught on camera unzipping Sinner's tennis bag and trying to take something.

A nearby security guard stopped him just in time, but it was still a terrible look:

Just inexcusable behavior.

Hopefully, the fan gets banned from U.S. Open events in the future and doesn't try to pull a classless stunt like that again.