At 6 foot 4, Yannick Noah can get to the net in two swift strides and smash the ball with one swift motion. It's not often he has to resort to finesse.

But last night against Mike Leach, a strong 5-11, 23-year-old who looks as if he were an all-America in football rather than in tennis, Noah had to rely on a good bit of guile to win, 6-4, 6-2, and advance to the third round of the D.C. National Bank Classic at Rock Creek Tennis Stadium.

In the evening's other feature second-round match, second-seeded Jose-Luis Clerc defeated qualifier Bernard Pils, 6-4, 6-1. Rodney Harmon of Richmond assured his best finish in Washington's summer tournament with a 6-1, 6-4 victory over Mario Martinez.

In an afternoon match played in 110-degree heat, Claudio Panatta scored a mild upset over 15th-seeded Thierry Tulasne, 6-4, 3-6, 6-3.

Leach, the 1982 NCAA singles champion, used astrong serve to stay in contention in the first set. Through the first five games, he scored every time he was able to get his first serve in and was able to build a 3-2 lead.

Mixing in a few obligatory drop shots, Leach kept the usually smooth Noah, the fourth seed in the tournament, off balance. But in the seventh game, Noah was able to handle Leach's serve long enough to break him and go ahead, 4-3.

Occasionally Noah countered power with power. Ahead, 5-4, in the first set, he used some hard serves--including one ace and one service winner--to take the set.

"He serves very hard and it's very hard to hit the ball hard back against a player like that," Noah said. "It (Leach's serve) is very impressive and it took me a while to get used to it.

"It is better to take the net before him. His ground Arthur Ashe on Drug Tests in Tennis--C2 strokes aren't as good as the rest of his game, so it's better just to hit the ball back over."

Noah broke the University of Michigan graduate again in the second set, taking the third game and then holding serve in the fourth to go up, 3-1.

Leach's frustration became apparent in the next game, when Noah lobbed his overhand smashes back to him three times before he was able to put the ball away.

About the same time, his high-charged serve went awry. He double-faulted on two straight points to give Noah the game and a 4-1 lead.

Noah held serve in the next game to expand his lead to 5-1. Leach avoided a fourth break to pull to 5-2, but Noah ended the set in the next game.

In the last game, Noah couldn't resist showing a little power. He took a lob close to the net and bounced it into the stands, afterward strutting around with his chest puffed out. Leach just watched. On the next point he treated a backhand lob the same way, driving the ball several rows into the seats behind Noah.

The fifth-ranked Clerc, who has won three tournaments in a row, encountered even less resistance against someone he thought was named Pilsner.

Through the first set he seemed tight, staying on the base line and volleying. He made serveral unforced errors in the first four games but still broke Pils twice to win the set, 6-4.

Clerc loosened up in the second set, joking with a ball boy and playing less cautiously in general. Pils tied the set at one game, and Clerc won the next five games. Over one two-game stretch, he didn't allow Pils a point as he assumed a 3-1 lead. He broke Pils' serve three times in the last set, including the last game, to preserve the 6-1 victory.

"I played much better, much more relaxed in the second set," Clerc said. "I got my confidence back and I didn't miss as many easy shots."

For Harmon, who played as a youngster on the courts here, last night's victory made more difficult his upcoming decision whether to turn pro. He is registered for another year at Southern Methodist University, but his recent successes have caused him to consider quitting school to go on the Grand Prix tour.

He was 25-5 this year under SMU Coach Dennis Ralston and has two years of eligibility left after transferring from Tennessee two years ago.

"I've playing smarter now. I'm a lot more consistent and I'm making fewer errors on the ground," Harmon said. "I'm showing people I can stay out there longer."

In second-round matches yesterday afternoon, eighth-seeded Mel Purcell rallied to defeat Tom Cain, 6-3, 7-5, and No. 6 Raul Ramirez had an easy time defeating Alejandro Ganzabal, 6-2, 6-4.

Other seeds to advance were No. 11 Van Winitsky, who defeated Gabriel Mattos, 6-3, 6-4, and No. 13 Jose-Luis Damiani, who defeated Ramesh Krishnan, 7-5, 7-5.

Now that the third and fifth seeds, Eliot Teltscher and Andres Gomez, have been eliminated, Ramirez seems a serious candidate for a spot in the final. No seeded players stand between him and a quarterfinal match with top-seeded Ivan Lendl.

The 18th-ranked Ramirez, whose victory in Caracas earlier this year was his first in two years, played in the final of the singles or doubles events in the Washington Star International, this tournament's predecessor, six times in the last seven years. In 1976 he lost to Jimmy Connors in the singles final.

Lendl was fined $850 yesterday for smashing his racket and hitting a ball into the stands during his second-round victory over Gilles Moretton Wednesday night.

The fine was the most severe levied by Ken Farrar, the Grand Prix supervisor here, for actions during a game. Earlier this week Gomez was fined $350 for hitting a ball after play was out and Eddie Dibbs was assessed a $500 fine for breaking his racket on the back of the umpire's chair.