Bill Murray: Luckiest Sports Fan of the Year!?

Bill Murray was all sorts of LIT after Xavier (11) knocked out Arizona (2) from the NCAA tourney Thursday night ... because his son is an assistant coach on the team!

Luke Murray has been with X since 2015 -- and his famous dad was right in the crowd as the Muskesteers took down the Wildcats at the SAP Center in San Jose.

Bill danced. He pumped his fist. He totally messed up his friend's hair.

Next up ... in the Elite Eight.

By the way, Murray is also a die-hard Cubs fan. So, yeah, he's having a good year.