Timothy Olyphant -- Pretty Badass Tennis Showdown ... Vs. Gavin Rossdale [Video]

Did You Know ... Timothy Olyphant and Gavin Rossdale are pretty good at tennis -- and the two battled it out in a furious rally at a celeb showdown in L.A. -- and we've got the footage.

It all went down at the Mak Games tennis tournament at Jimmy Goldstein's mansion in Beverly Crest -- where the stars teamed up with a couple of pros (Nicole Gibbs and Lester Cook) for a doubles match.

It's a pretty cool clip ... but despite winning the point, Olyphant ultimately lost match in 2 sets.