The former Boston Celtics forward joined the existing cast on the popular Fox show this week ... and it didn't take long for him to provide some hilarious laughs on set.

As he was getting familiar with Lance Armstrong's workout routine ... he noticed Tinashe was getting in some yoga on a couple of mats nearby.

Intrigued by the stretch sesh ... 45-year-old Pierce thought he might be able to do it -- and, spoiler alert, he couldn't.

Check out the clip ... while Pierce was kind of able to bridge his body similarly to the pop singer's -- he eventually began shaking, before he crumpled to the floor.

Everyone got a huge kick out of it -- including Pierce himself, who let out a big laugh.

While Tinashe made it clear she's wary of the show's newest castmates ... it's gotta be hard to freeze out Pierce going forward after that display of athleticism.