VIDEO: Tommy Haas sings along to Idina Menzel in his car

They say age is just a state of mind. Tommy Haas, who at 36 is the oldest player in the ATP top-100, and one of the oldest players in pro tennis, period, might agree. 

Well, lingering shoulder soreness forced the world No. 18 to retire in the first set of his first-round French Open match on Tuesday, despite a 5-2 lead. Maybe age is more than just a state of mind. 

But it's stuff like this that keeps you young at heart: here's Haas gamely singing along to Idina Menzel's Oscar-winning song 'Let it Go' in his car. 

The video was posted by Haas' wife, Sara Foster, whose father is music producer David Foster. Wrote Haas to his father-in-law: "any interest signing me? :) I know I know, I sound like I am taking a s%#!"