An aging tennis pro has got to hustle. The reflexes slow, the legs get wobbly and soon he's being aced out of all those locker room bets. But Gary Wilensky, 41, the director of two Manhattan tennis clubs, has developed a gimmick. He'll put on roller skates and play a set against anyone wearing sneakers who turns up on Central Park's asphalt courts. The former Grossinger Hotel instructor claims he began skating only three months ago, but devotes at least three hours a day to practicing tennis on wheels and gives lessons (at $16 an hour) and exhibitions. Wilensky has yet to be beaten but adds, "I have to be careful not to wheel fault."