It's Monica Seles - a former No. 1-ranked women's tennis player with nine Grand Slam titles.
The great ballman experiment ends with Kramer injuring Monica Seles in The Lip Reader - the 70th episode of the sitcom "Seinfeld".
See Yahoo Sports' top five tennis moments on 'Seinfeld'.
Episode Summary (from stanthecaddy.com):
Jerry is smitten with a line judge at the US Open, but after he gets her attention, he finds out she is deaf. Elaine has a problem not communicating with a limo driver. George wants to use Jerry's girlfriend to read the lips of his ex-girlfriend to find out what she is saying about him. While she is reading the lips, Kramer does the translation, that gets George in more trouble. Kramer becomes a ball man at the Open, when Monica Seles returns to tennis.
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