Lindsey Vonn Insists Just 'Friends' with Lewis Hamilton

Lindsey Vonn -- who spent her birthday weekend traveling to Austin to see Lewis Hamilton race -- is once again insisting the two are just "friends."

Okay.

The two got close, once again, at the F1 Circuit of the Americas ... a race Lewis dominated from start to finish.

Remember, she spent her birthday on a bowling date with the guy last week ... and last year, she visited the guy at the British Grand Prix.

Plus, just look at the picture they took together over the weekend -- that look like just friends to you?

The catch, however, is that Lewis went on "Ellen" last week and insisted he's very single and VERY focused on his racing this year ... and won't let a relationship get in his way.

That's great and all -- but dude, click the gallery below. Might wanna reconsider your policy.