Guillermo Vilas and Richardo Cano, the No. 1 and No. 2 men on the Argentinian team that upset the United States in the Davis Cup American Zone final this spring, advanced to the quarterfinals of the $125,000 Washington Star International tennis championships yesterday afternoon.

John Alexander, the No. 11 seed, upset No. 4 seed Roscoe Tanner, 7-6, 6-3, in a match started on the center court and moved to an outside court after the first set so that the stadium at 16th and Kennedy Streets NW could be cleared for the evening session.

Alexander lost his serve to fall behind, 1-2, in the first set, but broke back for 3-3. The match went on serve to 6-all, and then Alexander played a perfect tie breaker to win it, 7 points to 2.

The first five points went on serve, with both players getting all their first serve in, but then Tanner missed one and sent a backhand down-the-line wide as Alexander knocked a backhand return and followed it to the net.

Tanner then netted a high backhand volley, a bad error, for 2-5, and Alexander served out the tie breaker.

Alexander, 20, lost only one point in four service games in the second set and broke Tanner in the fifth game and again in the ninth for the match.

Vilas, the French Open champion who was runner-up to Harold Solomom here in 1974 and beat him in the 1975 final, was severely tested in a two-hour 34-minute match before beating Victor Pecci, the 6-foot-3 Leaning Tower of Paraguay, 6-3, 6-7, 6-1.

Cano, only No. 77 in the computer rankings of the Association of Tennis Professionals, outsed Californian Hank Pfister, 6-3, 4-6, 6-2, in a match ddominated by errors at windswept Washington Tennis Stadium.

An upset winner over Dick Stockton in the pivotal singles match in Argentina's 3-2 victory over the U.S. at Buenos Aires, Cano thus reached the quarters of a Grand Prix tournament for the first time this year, advancing through a section of the 64-man draw weakened by the withdrawal of No. 8 seed Arthur Ashe.

No. 2 seed Brian Gottfried, No.5 seed Eddie Dibbs and unseeded Ray Moore reached the quarterfinals Thursday.

Vilas and Cano, who have been the No.1 and 2 players in Argentina for several years will meet today. Gottfried will play Moore, and Dibbs will play the winner of a Friday night match between Bob Hewitt and Hans Gildemeister.

The final quarterfinal will pit John Alexander against the winner of the match between Harold Solomon and Phil Dent.

The crowd of 5,100 that watched yesterday's matinee session on a clear, sunny and comfortably breezy day - a marked contrast to the heat wave conditions in which the tournament's first four days were played - saw some fine tennis when Vilas, the No. 1 seed, played Pecci, the No. 14 seed who beat Solomon in last year's quarterfinals.

This way raw talent, in the form of the sturdy, massive-shouldered, 21-year-old Pecci, against the refined talent of Vilas the equally powerful lefthander whose quickness and topspin groundstrokes make him one of the most accomplished clay-court players in the world.

Pecci, who is exceptionally dangerous when he gets his big serve thumping, has enormous potential but he has not yet put it together. He is improving, but has not made the psychological breakthrough that separates champions from the mass of good players, that allows them to raise their games and play the critical points with assurance.

Pecci served for the second set at 5-4, but was broken as Vilas played a superb game, approcahing on a short ball and drilling away a forehead volley for 30-40, then breaking with a backhand, down-the-line oassing shot.

Pecci won the set in a tie breaker, 7 points to 3, with a beautifully angled forehand cross court volley after Vilas had double faulted and hit a bad forehand error off a wind-blown lob on the two points he served from 3-4.

But instead of bearing down right at the start of the third set, Pecci played a terribly loose game to lose his serve at 15 in the first game. He started with a double fault, hit a bad drop shot wide and, after saving one point, netted a forehand volley and pushed a backhand approach long.

Vilas held his serve at love and then bore down for the kill the way Pecci had not. He got 0-30 by crunching an overhead after getting to the net behind a deep forehand down-the-line approach, then knocked a backhand down-the-line winner in the corner off a drop shot.

From 30-30, he played two even better points. First he hit a lunging forehand cross-court volley winner, displaying remarkable quickness and anticipation as he spun around the net like a whirling dervish for three volleys and a backhand smash in a rapid fire exchange. Then he applied the clincher with a backhand down-the-line passing shot as he ran, back to the net, to cover a lob in the corner.

That made it 3-0, two service breaks, and Pecci was for all intends and purposes finished.

"I know he's a very good player and I was a bit afraid of him. He serves very well when he gets his serve going," Vilas said.

"But he was playing very risky in the second set. He was playing well, but very near the lines. He was playing a lot of drop shots, which is very difficult on clay. He could not keep it up in the third set.

"He plays a very strange game on clay," Vilas added. "Sometimes he serves and volleys, and sometines he just puts the ball back and stays on the baseline. I think he has all the things to be a very good player, but he has to learn how to use them. But he's only 21."

Cano, 5-foot-6 and 154 pounds, grew up on clay but likes to play the net. He went in in everything when he had the wind at his back, and control over his soft volleys ultimately prevailed over Pfister's much harder but more erratic volleying.