Top-seeded Ivan Lendl was bounced out of the Washington Star International tennis tournament yesterday, losing his quarterfinal match to a relatively unknown 20-year-old Ecuadorean, Andreas Gomez.

And in another shocker at Rock Creek Stadium, unseeded Stanislav Birner, a determined Czechoslovakian, upset 12th-seeded Mel Pucell, 1-6, 6-3, 6-1, for his seventh victory of the tournament. Birner began this $200,000 event in the qualifying round.

In today's first semifinal, at 1 p.m., Birner will meet Jose-Luis Clerc, seeded third, who last night defeated Raul Ramirez, 7-6, 6-2. The evening semifinal, at 7:15, will match Gomez against fourth-seeded Guillermo Vilas, a 6-2, 6-1 winner over Mario Martinez.

Gomez had plenty of impetus for his 2-6, 7-6, 6-4 victory over the fourth-ranked player in the world. His home city of Guayaquil, Ecuador, 443 years old yesterday, was celebrating its independence day.

Gomez, who knew his countrymen were awaiting the result of his match, was nervous in the first set, allowing Lendl to blast him into submission with forehands.

The 6-foot-5 Gomez fell behind, 2-4, in the second set and it seemed the only thing in question would be Lendl's margin of victory.

"But I knew I still had a chance," Gomez said, "because Lendl had beaten me, 6-2, in the first set of the French and I had come back to lose to him in five sets.

"In the second set, I decided to keep the pressure on him and to go for winners on the big points. He then made a lot of unforced errors. His game doesn't bother me that much."

So the big left-hander steadied himself at the base line, served and volleyed wtih uncommon poise and played as many shots as possible to Lendl's backhand to avoid the cannon-shot forehands that had crushed him in the first set.

Gomez held serve at 3-4, then broke back to tie, 4-4. He held again to take a 5-4 lead a the Stadium Court underdog lovers cheered. Each player held serve the next three games to force a tie breaker at 6-6. In the tie breaker, Gomez initiated the action and made the correct choices on when to come to the net and when not to, winning, 7-1.

In the third set, Gomez took a 2-0 lead, but Lendl caught him at 3-3 by breaking Gomez at love. As "fold" and "choke" were being whispered, Gomez was breaking back for a 4-3 lead.

Gomez, ranked 71st in the world, scored aces on the first and fourth points of the 10th game to win at love and evoke an ovation from the 3,000 in the stadium on a gray, drizzly day that saw matches delayed three hours because of rain.

"This is a good gift for the people of Guayaquil," Gomez said. "Right now, this is the biggest win of my life."

Lendl, asked about his many unforced errors in the second and third sets, replied, "If you think they're unforced, why don't you try picking up a racket and hitting them back."

As Lendl left the court, countryman Birner was coming on to play the heavily favored Purcell. Birner, ranked 153rd in the world, did almost nothing right in losing the first set.

But Purcell, who had become the crowd favorite, seemed to lose interest and concentration. He let a controversial line call on his service early in the next set bother him the rest of the match while Birner was improving every phrase of his game: base line strokes, volleys, service and movement.

"I went flat," Purcell said. "I haven't yet learned how to settle down after a line call upsets me. But I get mad when I'm ripped off and cheated. She (the lineswoman) wouldn't even come out and look at the mark on the court. That's taking $1,000 out of my pocket. I was so mad, I just couldn't come back. It was the biggest point of the match."

While Purcell was fuming, the always calm Birner "attached his backhand. It's not too good." There were shouts of "Let's go, Stan!" from the grandstand and Birner said he heard every voice. "I've never had support from the fans in the United States," he said. "It felt so good."

In what was probably the most exciting set played so far, Clerkc, the No. 3 seed, defeated the still-fluid, shotmaking Ramirez in a dramatic tie breaker, 10-8. Ramirez fought off three set points and had one himself before Clerc prevailed.

"I didn't expect that much trouble from Raul," said Clerc, ranked No. 5 in the world."I was surprised he played and moved so well. I played him earlier in the year and had no trouble."

Vilas, who won his fourth straight match in straight sets, has yet to play a seeded competitor.