I'm Going to TennisWorld - Clue #3

Ok, folks, here's the third and final clue to the Tennis Magazine U.S. Open Trivia Challenge (see my previous posts, starting with I'm Going to TennisWorld!, for details):

The U.S. has a long and distinguished tradition in doubles. The two players on this U.S. doubles team combined to finish as runner-up in seven Grand Slam singles competitions.

Please submit your answers to TennisWorld@tennismagazine.com

No other form of entry will be accepted. You may enter as often as you like. The winner will be annouced on Monday, August 14th. Good luck!