The lovely couple were sitting together at the tennis final Sunday when they were both seemingly overtaken by the music ... launching into their own renditions of "I Wanna Dance with Somebody" by the late, great artist.

You can't hear the pair singing, but just check out the clip ... the dynamic duo's clearly putting on quite the show for anyone looking their way. TH is even getting his body rockin' to the beat.

And, the end of the singing is just as good too ... with the couple sharing a joke during the last notes of the song -- not totally clear what they're laughing about, but they look adorable here, seemingly lost in their own world

The pair were dressed in their tennis best ... a good thing too, 'cause Zendaya headed down to the court after the match to take a snap with the women's champ Iga Swiatek.

What a day for Iga -- a trophy and a pic with Zendaya ... no wonder she's grinning from ear to ear!

BTW both Tom and Zendaya have their own affinities for the sport -- TH has been photographed at multiple tournaments, just like Zendaya ... who seems to like it so much she's starring in the movie "Challengers" based around the sport.