Guillermo Vilas and Harold Solomon, who met in the finals in 1974 and 1975, won their second-round matches in the $125,000 Washington Star International tennis championships yesterday over two more lefthanders in the impressive wave of 17-to 22 year-old American players.

Vilas, 24, the French Open champion and No. 1 seed, outstroked and exhausted, 6-4, 6-1, a youngster who idolizes him, 18-year-old Wimbledon Junior champ Van Winitsky.

Solomon, 24, the No. 6 seed who won the title here in 1974 and was runnerup to Vilas the following year, never had a break point against him during a 6-4, 6-3 victory over Bruce Manson, 21, who will be a senior at the University of Southern California in the fall.

Eddie Dibbs, the No. 5 seed, reached the quarterfinals by eliminating, 6-3, 6-4, another lefty who hits with heavy topspin, Terry Moor.

Dibbs had his own way of combating the heat and humidity that again enveloped Washington Tennis Stadium. At change-games he gulped from a portable oxygen tank given to him by a friend last week in Cincinnati, where the Western Open was played in similarly oppressive mugginess aggravated by danger-level pollution.

Ray Moore of South Africa, unseeded but a semifinalist here last year, joined Dibbs in the quarters with a 7-5, 3-6, 6-1 victory over Tim Wilkison, 17, of Shelby, N.C., who decided to bypass college and turn pro.

Hank Pfister, 23, a rangy 6-foot-4 Californian from San Jose State, registered one of his better clay-court victories by beating Zeljko Franulovic, 3-6, 6-3, 6-4. Franulovic was the U.S. Clay Court champion in 1969-71 and runner-up in the French Open before a series of shoulder injuries nearly ended his promising career.

Phil Dent (seeded No. 9) and John Alexander (No. 11), powerful Australian Davis Cup teammates who have become formidable clay-court players in the last year, advanced to the second round in straight sets.

Dent, 27, a semifinalist in the Italian and French opens and a quarterfinalist at Wimbledon this year, dispatched fellow Aussie John James, 7-5, 6-3.

Alexander, 26, who proved that he can play on clay by winning both his singles in Australia's 3-2 loss to Italy in the Davis Cup semifinals at Rome last fall, beat plodding 6-foot-5-inch lefty Victory Amaya, 6-2, 6-4.

"I feel I'm playing better tennis right now than I ever have, and so I expect eventually to have better results than I've ever had," said the sturdy 6-foot-3 Alexander, who was hailed as the next Aussie, Superstar at 17 and may have been weighted down by the great expectations of his countrymen.

Last week, in his second clay-court tournament of the year, Alexander took Solomon to 7-5, 7-5 after having a set point in the first set. He beat Solomon on clay earlier this year at Charlotte.

"I'm becoming a more complete player. It's a long process, learning this game of tennis, but I've worked hard to eliminate some weaknesses," Alexander said.

On the low volleys, I tended to push the ball back and not do enough with it. On high volleys, I tried to be too aggressive and consequently hit the ball so hard that it bounced high enough to let the opponent run it down."

"On clay, a bad volley is a good shot for your opponent to pass you on, so I've really worked on putting volleys away on this surface, firming up the low ones and angling the high ones."

Australians generally learn a "grass court volley" - punched deep rather than angled away because the ball stays low and shoots through quickly on grass - and have to develop the "clay-court volley," Alexander said. Both he and Dent now have doen so.

"Phil and I had a long talk before he went to Europe," Alexander said. "You have to look at it this way - from the baseline, we are almost as steady as the clay courters, but we have the advantage of being natural volleyers. If we can just be patient and wait out the right opportunities to approach, we should win because we're better at the net.

"I think that in the and, after the long maturing process, Australians are better all-around players than guys who play only on clay or only on fast courts," said the positive-thinking Alexander.

"Look at Ken Rosewall. At 17 he was a baseline player, but he learned to serve-and-volley out of necessity, to survive on fast courts. Harold Solomon may never become a complete player because there are so many tournaments on slow courts he might never have to learn to serve-and-volley."

Solomon, who has won 11 straight matches the last two weeks, today had the rare experience of never being in danger of losing his serve. He lost only 10 points in nine service games.

But this was hardly one of his memorable matches, even though he served well. "This is lullaby time. Everybody's going to sleep," said Solomon's girlfriend, Jan Lindsay, in the middle of the second set. "They're both making so many errors it's a joke."

"It wasn't real good tennis, obviously," agreed Solomon. "I was bored playing it. Neither of us was hitting the ball very well."

Vilas had never before seen Winitsky, who grew up in Florida and will be a sophomore at UCLA, having skipped his senior year of high school. Winitsky grew up on clay courts in Miami and Fort Lauderdale but has learned to be more aggressive by playing on cement on the West Coast.

"Vilas has been my idol since Rod Laver retired. I had to have a lefty, and I like the way he hits his topspin and plays on clay. It was kind of tough going out to play him," said Winitsky, who stuck with Vilas in torrid rallies in the first set, then tired badly and saw his game collapse in the second.

"I was so tired I just couldn't move. I needed Dibbs' oxygen machine," said Winitsky. "He's got a couple of years on me. He's stronger and in better shape. He's more experienced."

Vilas said he sees little resemblance between himself at 18 and Winistsky, because Winitsky serves and volleys better, mixes some underspin with his topspin and is not as solid from the backcourt. But others saw much in common.

At times in the first se they looked like mirror images of each other, until Winitsky lost his serve in the 10th game to lose the first set, then drained perceptibly.