I'm Going to TennisWorld! - Clue #1

Okay, folks, here's the first clue to the Tennis magazine U.S. Open Trivia Challenge (see my previous post, I'm Going to TennisWorld!, for details):

The U.S. has a long and distinguished tradition in doubles. This doubles team was one of the relatively few U.S. Open-era doubles teams to win a major clay court championship.

Please submit your answers to TennisWorld@tennismagazine.com

No other form of entry will be accepted. You may enter as often as you like. Good luck!