Rafael Nadal -- Serves Up Girlfriend's Ass ... During Cancun Vacation

The perks of being the #1 tennis player in the world ...

With just days until he's scheduled to hold court at the BNP Paribas Open in California, Rafael Nadal was chillin' in Mexico with his hot girlfrend Maria Xisca Perello.

The two were seen kayaking together on Tuesday -- the tournament begins on Thursday ... though Rafa has a first round bye since he's a #1 seed.