Until yesterday afternoon, Manuel Orantes, the elegant left-hander who won the U.S. Open in 1975, never had heard the name Fritz Buehning.

Now, much to his distress, he knows it quite well.

Buehning, a lanky 19-year-old from Short Hills, N.J., who had win three qualifying matches to reach the first round, recovered from a shaky first set yesterday and stunned fifth-seeded Orantes, 0-6, 6-3, 6-4. The Spaniard was the third seeded player eliminated in the first round of the $175,000 Washington Star International Tennis Tournament.

Top-seeded Guillermo Vilas also had some problems with another little-known amateur, 20-year-old Jay Lapidus, a Princeton undergraduate.

With the evening crowd of 4,500 cheering him on, Lapidus took the first set into a tie breaker. Then, in a battle of nerves, he matched the Argentine left-hander with one top-spin after another until the score reached 5-5.

The next point lasted 70 hits and was the turning point of the match. Lapidus finally hit a backhand off the wood wide. On the next point he missed another backhand and Vilas had the set. He cruised through the second set for a 7-6, 6-1 win.

Third-seeded Harold Solomon had an easy time with Haroon Ismail, 6-2, 6-3. Eight-seeded Victor Pecci also won easily, over Eric Iskersky, 6-3, 6-4.

Second-seeded Eddie Dibbs, looking extremely strong coming off his win Sunday at Forest Hills, defeated Terry Moor, a left-hander from Louisiana, 6-3, 6-4.

While the evening crowd enjoyed watching Vilas struggle in the first set, is was the afternoon audience that witnessed the tournament's biggest upset to date.

Orantes was not the only one in the steamy Rock Creek Tennis Stadium who never had heard of Buehning before he took the court yesterday. An amateur ranked 350th on the ATP computer, his biggest win previously had been a first-round Wimbledon victory over Richard Meyer.

When Orantes breezed through the first set in 21 minutes it did not look like too many in the crowd of 3,000 would remember his name much longer.

"The first set I didn't really play that badly," said Buehning, who does not shave before big matches.

"I knew I could play with him if I could get into my serve-and-volley game. Once I calmed down I began doing that much better and he began to miss some at the net."

Down, 3-1, in the second set, Buehning began knocking service winners at Orantes and passing him when Orantes was serving.

"The first set was so easy it was like practice," said Orantes. "All of a sudden he started serving much better and I couldn't get my concentration back. Then in the third set he just played better than me on the big points.

Orantes was one of two seeds who did not make it past the first round yesterday. But the other one, 12th-seeded Johan Kriek, exited by choice. Kriek, a 20-year-old South African, was married Saturday and had no intention of appearing here.

However, he failed to inform tournament officials and he was scheduled to play at 1:30 p.m. against Rejean Genois. Thirty minutes before the match, officials realized that Kriek was not going to show. Juan Nunez of Chile subbed for Kriek and ebat Genois, 6-1, 4-6, 6-4.

Three seeds did survive in the afternoon heat - measured at 100 degrees courtside. Mexico's Raul Ramirez, saying he felt on top of his game after a prolonged slump, looked extremely sharp, swamping Australian Ross Case, 6-2, 6-3.

At the same time, fourth-seeded Jose Higueras was having almost as easy a time with hard-serving John Sadri, a 1978 graduate of North Carolina State, winning 6-3, 6-4.

Like Higueras, Vilas used a steady back court game to wear down his inexperienced opponent. "He (Lapidus) plays the serve and volley very well," said Vilas. "He was very tough to attack because he was also passing well. I stayed back because I realized that was the strategy that would win."

Vilas, a second-round loser at Wimbledon and a first-round in loser in Switzerland last week, said he is pleased with his game. "I'm hitting the ball well," he said. "I just need to play a lot of matches to get my confidence back."

The easiest match of the day for a seeded player was sixth-seed Jose-Luis Clerc's 6-4, 6-1 win over Sherwood Stewart. Stewart, one of the world's best doubles players, could not keep up with Clerc, who at age 20 already is ranked 12th in the world.

The two most competitive matches of the afternoon involved unseeded players with the crowd favorite losing in the end.

First, Jim Delaney, from Potomac, lost a seesaw battle with Bruce Manson, 2-6, 6-4, 7-5. Delaney dominated the match early, leading 3-1 in the second set, then fell behind a break in the third and battle back to 5-5 only the watch Manson rifle two passing shots while down 5-6 and deuce for the match.

"At the beginning of the match, I didn't think he was ever going to miss a shot," said Manson, who earlier this year beat Bjorn Borg in Palm Springs, Calif. "Everything he tried worked. I had to play a lot more agressively than I thought I would have to." CAPTION: Picture, Fritz Buehning awaits serve in win over Manuel Orantes. By Margaret Thomas - The Washington Post