Martina Navratilova Proposes to GF During U.S. Open ... She Said Yes!

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Tennis legend Martina Navratilova is finally gonna make an honest woman out of her girlfriend ... dropping down to one knee Saturday at the U.S. Open and popping the big question.

The 57-year-old proposed in between the men's semi-final games to Julia Lemigova -- her GF of 6 years -- and people in attendance cheered on as they watched the proposal on the big screen.

Martina told the AP she was super nervous ... describing it as an "out of body experience."

And you thought being on the Kiss Cam was romantic.