Redfoo -- Training to Be Tennis Pro ... Everyday, I'm Hustlin'

He may look like he's goofin' around, but Redfoo from LMFAO says he's deadly serious about a career as a PRO TENNIS PLAYER ... and he's been training his ass off to make it happen.

Here's the deal ... Redfoo has been a hardcore tennis fanatic for years -- and even dated former #1 women's player Victoria Azarenka for a few years.

And over the past few years, Redfoo says he's been studying the top players and putting in work -- and is 100% determined to be taken seriously as a pro player.

Check out the clip -- you can tell Redfoo means business.

The only problem is he's 39 -- old for an "up and coming" pro athlete ... especially since he failed to make the cut in a U.S. Open qualifier back in 2013.

But hey, anything's possible ... right?