Steve Clarke, Washington's top-ranked junior last year, knocked off Mark Notttingham, 6-0, 6-1, in the 18-and-under division of the Greater Washington Boys Tennis Tournament and returned at noon to defeat Edward Cruzat, 6-2, 6-3, for the 21-and-under crown yesterday at Washington Tennis Stadium.

Clarke 18, is a stocky, 5-foot-11 high school All-America who graduated this year from Fork Union Military Academy near Richmond. He used a high-kicking spin serve adn wristy, looped ground stroked to take the first set fromCruzat, who lives in Chicago.

Clarke dropped only four points on his serve in that set and had little trouble outsteadying his opponent on slow, dry Har-Tru court.

But Cruzat settled down, starting blasting winners from the net and won 12 of the first 14 points of the second set to move ahead, 3-0.

"In the next game," said Clarke, who went 53 straight matches without a loss in his last two years of high school, "Cruzat had four five ads which could have put him up 4-0. Early in the set I'd been missing my shots by a few inches and in that long game, which I won, my rhythm returned.

"Once I got close, I knew I'd get back into the contest and I took the next five games to end the match," added Clarke,. who lives in the District and practices daily at the 16th and Kennedy streets courts.

In a clash of future stars, Jeff Hersh of Rockville outdueled Joseph Blake of Cumberland, Md., 6-4, 6-2, for the 12-and-under title. But Blake got revenge later on, defeating Hersh by the same scores for the 14-and-under championship in one of the better-played matches of the day.

Steve Ornstein defeated Brett Schaffer, 6-1, 6-2, for the 16-and-under title and Wade McGuire upset top-seeded David Kanstoroom in the 10-and-under division. CAPTION: Picture, Jeff Hersh, 12, ponders a point during Greater Washington event.