Guillermo Vilas, Brian Gottfried and Eddie Dibbs - the No. 1, 2, and 5 seeds - reached the semifinals of the $125,000 Washington Star International tennis championships yesterday afternoon, defeating unseeded opponents who had come through sections of the draw weakened by earlier upsets and withdrawals.

Vilas, 24, the muscular lefthander who won the French Open last month and is among the top three clay-court players in the world, never had to accelerate above cruising speed to beat his Argentinian Davis Cup teammate, Ricardo Cano, 6-2, 6-3, in a tedious match.

Gottfried, 25, the Colgate Grand Prix leader who has been in the finals of eight tournaments this year, including the French, and won four of them, eliminated Ray Moore, a semifinalist here last year, 6-3, 6-4.

Dibbs, 25, ousted the erratic and unorthodox Hans Gildemeister 6-4, 4-6, 6-1, in perhaps the strangest match of the week. Gildemeister led 4-0, in both the first and second sets.

No. 6 seed Harold Solomon, who beat Vilas for the title in 1974 final and lost to him the 1975 final, was scheduled to play Australian John Alexander last night.

Gottfried plays the Solomon-Alexander winner in the first match of today's afternoon session at 2 p.m. Vilas plays Dibbs for the first time this year they split tough matches last year, Dibbs winning in the U.S. Pro Championship and Vilas in the U.S. Open - in the evening opener at 7 p.m.

The matinee audience of 5,300 at the Washington Tennis Stadium yesterday saw the tournament pay for earlier upheavels with lopsided quarterfinal pairings.

Cano had emerged from the quarter of the draw softened by the withdrawal of Arthur Ashe: Moore from the quarter in which Stan Smith and Cliff Richey were first-round victims; and Gildemeister from the section in which Raul Ramirex was upset and Jaome Fillol pulled out with a bad cold.

After the thrilling Friday night fight in which Solomon beat cramps and Phil Dent. 6-7, 6-4, 7-6, and then collapsed in a heap of rebelling muscles at the end of the excruciating final set tiebreaker, yesterday afternoon's program seemed as exciting as mowing the lawn.

Moore-Gottfried could have been a good match, but neither played very well until Gottfried, down 0-40 in the eighth game of the second set, began to play the attacking game that is his natural style. Lethargic earlier, he won 15 of the last 17 points.

Vilas, who has been playing and beating Cano since they were 14, did so again in a match in which far more points were lost than won. "If this were a Broadway show," suggested one critic, "it would be closing tonight."

It was Gildemeister, 21, the fourth man on the two-man Chilean team that reached the final of the Davis Cup last year, who surprisingly provided the most resistance, and thus the most entertainment.

An improving player who went to school at the University of Southern California and now is No. 73 in the computer rankings of the Association of Tennis Professionals, he hits double-fisted both forehand and backhand, though he usually volleys one-handed on his infrequent trips to the net.

Tall and smiling, with long hair and a kind of natural flamboyane, he brought the crowd to life by whacking a stream of winners to jump to a 4-0 lead in the first set against Dibbs, the Miami hustler who had never played him before.

But suddenly, the magic deserted him, almost totally. The buzzing groundstrokes started finding the middle of the net, or straying several feet wide. The drop shots didn't work. The first serve went off. The occasional volley was an atrocity. Dibbs, thumping his cross-court forehand and two-fisted backhand, reeled off six games in a row for the set. Requiescat, Gildemeister.

But not quite yet. Much to the crowd's astonishment and delight, young Hans rubbed his magic racket handle again, and the winners came sailing off his strings again. Once more he broke Dibbs two times for a 4-0 lead.

When he promptly played two terrible games for 4-2, it seemed that the second set might be a carbon copy of the first. "This guy is like a sprint horse in a mile-and-a-half race. Great for the first six furlongs and then, proof." chuckled Frank Defort, who will be able the play-by-play commentator for the Public Broadcasting Service's telecast this afternoon (WETA-TV-26 at 2 p.m.

But Gilremeister held his serve at love with a blisthering backhand down-the-line winner and then broke Dibbs for the set with a backhand cross court on the baseline as the crowd roared its approval.

Gildemeister lost his serve to start the final set, however, netting a backhand after two forehand passing shots - the first off a drop shot that didn't drop - put him at 15-40.

This time Dibbs rolled to a 4-0 lead. Guildemesiter held his serve for 1-4, but Dibbs ran out the set and the match.

"He plays well. He changes the pace every shot, so it's difficult to get grooved against him. He puts a lot ot topspin on hte ball, throws up some good lobs, mixes it up," said Dibbs. "He's going to be a good player."

Only tow other players on the procircuit. Frew Mcmillan and Gene Mayer, hit two-fisted from both sides, which allows them to hit powerful and well-disguised shots. "But it definitely restricts their reach a lot. If you get them stretching all the time, you've got them, said Dibbs. "If they can get in position to take a crack at the ball, they can do pretty much whatever they want with it."

Gottfried dueled Moore in long, straightforward backcourt rallies for much of the match, which was a tactical mistake on his part. Both of these players are sold - Moore has improved his formerly vulnerable forehand and Gottfried, thoug bland as a personality is a joy to tennis purists in this age of unorthodox shotmakers - but the name of Gottfried's game is aggressiveness.

After losing a 2-0 lead in the first set, he broke Moore for 5-3 by working in for a backhand volley winner at 30-40, and served out the set. But then he curiously reverted to baseline exchanges, forgetting to exploit his forte: the good serve, approach shot and volley.

"I began hanging back again, waiting for him to make the error instead of attacking and making him hit the good passing shot," said Gottfried."We had a long first game (Moore held serve after eight deuces and four advantage points against him), and after that I stopped thinking for awhile. Maybe that's what the sun beating down on your head does."

Gottfried remembered to attack when Moore had him 0-40 in the eighth game, on the verge of a break that might well have meant the set. He served and volleyed well to get out that hole, and in taking the last two games.

Vilas, whose topspin groundstrokes have much more pace than Cano's, lost his serve three times during the match, but never was in any danger. Cano's backhand was wretched throughout, and he never could apply much pressure from the net.