Tim Tebow's taking his upcoming wedding as serious as one of his workouts ... 'cause the ex-Heisman Trophy winner was sweating his face off practicing his moves with his fiancee!!
The former NFL first-round pick showed off his waltzin' skills with Demi-Leigh Nel-Peters ... and he also flashed a healthy sweat that could be seen through his pink shirt.
Don't worry ... Demi-Leigh -- Miss Universe 2017 -- didn't seem to mind the perspiration ... in fact, she shouted out the guy for improving!!!
"Getting better," she wrote .