Seth MacFarlane -- I'm No Sinatra ... But That Ain't Stopping Me

Seth MacFarlane bringing Frank Sinatra back to life with a spot-on performance ... is NOT what happened in a piano bar the other night. Instead, Seth put down a pretty funny version of a Blue Eyes classic.

Seth got on the mic at Lucille's in Milwaukee last Saturday night and belted out a few Sinatra songs, including "Luck Be a Lady." The "Family Guy" creator forgot a few lyrics, but made up for it with a few expletive-laced improvs.

We're told Seth hung out with a group of friends for 7 hours ... and dropped about $1K on food and drink.

Fun Fact: Seth sang with Frank Sinatra Jr. for a 2006 "Family Guy" episode ... with much better results.