Tennis Star Milos Raonic Warms Up for Tourney ... With Smokin' Hot Model GF

It's sunny and 75 in Miami ... so tennis pro Milos Raonic decided to hit the beach with his smokin' hot model girlfriend, Danielle Knudson, before his big tennis tourney this week.

Milos is in town for the Miami Open -- where he could face off with Rafael Nadal (if they both get far enough) ... so why not blow off some steam on the beach, right?

If Knudson looks familiar ... she's a pretty big model and also had a brief fling with Justin Bieber back in the day.