The U.S. Tennis Assn. reached its lowest point ever Saturday when three overgrown brats made it to the men's semifinals. First, there was the spoiled John McEnroe, whose temper tantrums are legendary. Then it was Boris Becker, who is trying to copy McEnroe. Finally, there was the strange Andre Agassi, who borrowed his sister's clothes to play in. She plans to wear them at Halloween.