Prior to his semifinal match in the men's 70 national clay court tennis championships yesterday, unseeded Bill Foulkes made a couple of predictions. "I'm going to give it my best, but I don't think I'm going to win," he said about his chances against the legendary Gardnar Mulloy.

Sitting next to him was fourth-seeded Alex Swetka. " . . . But I think Alex may win this whole thing," Foulkes said.

At least half of those words proved prescient as he was beaten by top-seeded Mulloy, 6-1, 6-4, and Swetka surprised No. 2 Fritz Klein, 7-5, 6-4, at Army Navy Country Club in Arlington.

Mulloy, who has won the title the last four years, faces Swetka in the final today at 10 a.m.

In yesterday's doubles final, Mulloy and Verne Hughes won their fourth straight championship with a 6-4, 6-1 victory over Klein and Joseph Dunn.

"His {Mulloy} drop shot was spectacular," said Foulkes, who is from Reisterstown, Md. "I'm pretty quick, but, geez, I couldn't get them. And then I had some nice drop shots and the son of a gun got to them."

Mulloy, 74, is the all-time leader in USTA national championships with 79. He won the Wimbledon doubles title in 1957, was a member of the U.S. Davis Cup team for eight years and was the top American player in 1952.

"He {Foulkes} played better in the second set," said Mulloy, who was born in Washington and now lives in Miami. "I tried to back him up and keep him close to the baseline and use the drop shot."

Foulkes led, 4-3, in the second set, but Mulloy won the last three games. After a long rally, which included numerous overheads, lobs and base-line shots, Mulloy clinched it with a perfect drop shot.

In the first set, Foulkes broke Mulloy's serve in the second game to even the set at 1, but then he was broken three times as Mulloy won the set easily. Foulkes dropped the first two games of the second set, but won four of the next five games to take a 4-3 lead.

Foulkes was the surprise of the tournament with upsets over third-seeded Hughes and No. 7 Mickey Pardew of Towson, Md.

In the other semifinal, Swetka won the last two games of the second set to advance to the championship. Swetka is no stranger to title matches: He won the clay court world championships held in West Germany in June.