Jon Hamm Snags Loose Ball, Pisses Off Tyrese Haliburton During Game 1 At MSG

The Knicks had an extra hand in their thrilling Game 1 matchup with the Pacers in the Eastern Conference Finals on Wednesday ... but unfortunately for the home team, Jon Hamm's assist couldn't help them to victory.

The "Mad Men" actor had courtside seats for the contest at Madison Square Garden ... but Hamm apparently wasn't satisfied with sitting back and observing -- 'cause in the second quarter, he took matters into his own hands when the rock came flying in his direction.

Pacers star Tyrese Haliburton seemingly felt like he could've saved the ball to keep it in play ... and rushed to Hamm's section in hopes of deflecting it back onto the court.

But Hamm took swift action and snatched it ... resulting in Hali having a mini tantrum and calling for fan interference.

The ball ended up belonging to the Knicks ... but the game went to the Pacers, who pulled off an unbelievable fourth-quarter comeback thanks to Aaron Nesmith's three-point clinic -- and Indiana came out on top in overtime, 138-135.

Hamm was one of several big names at the arena for the game ... including Timothée Chalamet, Suni Lee, Ben Stiller, Larry David, Livvy Dunne, Martha Stewart and plenty more.

Someone who wasn't?? Haliburton's dad, John ... as he was told to stay home from the rest of the playoffs due to his on-court interaction with Giannis Antetokounmpo in the first round.

Things could change on that front, though ... as even Charles Barkley advocated for his return to NBA commish Adam Silver -- saying he learned his lesson.