You say you want a sure tip on who's going to win the men's singles in the U.S. Open tennis championships? A fail-safe bet. A lock.

Well here it is. The champ will be an American. Absolutely. Without a doubt. You can bet on it.

For the first time since 1950, when champ Art Larsen beat Dick Savitt and runner-up Herbie Flam zapped Gardnar Mulloy, there are four U.S. men in the semifinals of America's premier tennis tournament. Defending champion Jimmy Connors plays John McEnroe and Vitas Gerulaitis plays Roscoe Tanner in the semifinals Saturday (WDVM-TV-9, noon).

So who's going to win?

Better you should ask Andrew Beyer, our fearless handicapper, who would calculate the speed of Tanner's serve at mid-afternoon, divide by Gerulaitis' relief that Bjorn Borg is out of the tournament, factor in the effect of McEnroe's having played only one singles match in a week, and multiply by Connors' deep-seated belief that he owns this title.

From this he would figure the best odds, couple his choice with Chris Evert in a "double," and spend the next week explaining what went wrong.

The fact is that this year's semifinals, somewhat unusually for a major tournament, offer two fascinating matchups and four players who all truly believe this is their year to win.

Connors, who was seeded No. 2 behind four-time French Open and Wimbledon champion Borg, is the defending champion. He has been in the final five consecutive years, something no man has done since Bill Tilden in the '20s, and won in 1974-76-78. The asphalt-based hard courts of the National Tennis Center are his favorite surface. Even though he has not been especially sharp through the quarterfinals, with Borg gone he is the logical favorite.

But . . . McEnroe has improved vastly since Connors demolished him in last year's semis. He is 2-6 lifetime against "Jimbo," but 2-2 this year. He won their last meeting decisively, 6-1, 6-4, 6-4, in the semis of the World Championship Tennis Finals at Dallas in May and no longer has a psychological block playing Connors.

"When I first go to the semis at Wimbledon, I didn't think I had a chance against him," said McEnroe, recalling 1977, when as an 18-year-old qualifier he became the youngest semifinalist in the history of tennis' oldest tournament. "I think that set me back quite awhile because each time I played him I didn't really think I'd win, until about the fourth or fifth time. Now I've gotten over that. I've beaten him a couple of times, so I know I can do it. I play him like anyone else."

McEnroe won his third-round match over Englishman John Lloyd and his quarterfinal over Eddie Dibbs by default. But he doesn't think he will be rusty because he has been playing doubles. "Mac the Knife" and partner Peter Fleming added the U.S. Open title to the Wimbledon crown they won in July by beating fellow Americans Stan Smith and Bob Lutz in today's final, 6-2, 6-4.

"I like playing doubles, and I think it helps your singles because you can work on some things . . . You're serving and volleying all the time in doubles, and that's a lot of my game in singles," said McEnroe, 20, the left-hander from nearby Douglaston. Long Island, who is at home on these hard courts.

Tanner, the No. 5 seed, is the most popular among his fellow semifinalists for having eliminated Borg. Even though he won't have the advantage of playing under lights in the semis, which would help his 140-mile-per-hour serve, he has proven himself a much more well-rounded player than he used to be.

He returned served exceptionally well and hit some blazing passing shots against Borg. He also is moving better, in the backcourt and at the net, and is less inclined to lose his temper on the court. And he has expanded noticeably in confidence after taking Borg to an excruciating fifth set in the Wimbledon final and beating him here, matches that he says have changed his life.

"Any time you play the No. 1 player in the world in the No. 1 match in the world and you fight him to five sets and have a chance to win, it certainly makes you realize you can play in that category," Tanner said. "You know you can do it. You don't feel like you're in an experimental area."

Tanner will be jumping on every second serve and getting to the net behind his return, just as he did against Borg, attacking the most vulnerable part of Gerulaitis' quick and flashy game. Tanner is not worried about a letdown after his masterful performance against Borg.

"This is the U.S. Open, not the Chattanooga Open," said the native of Lookout Mountain, a suburb of that Tennessee city. "You don't have a letdown in the U.S. Open."

But Gerulaitis will be much happier playing Tanner, whom he has beaten in three of four meetings, than Borg, against whom he is winless in 13 tries. He must have danced a fandango when Borg lost, even though he likes the Swede and realized immediately that he was "heartbroken."

Gerulaitis, who also will be trying to get to net as often as possible, respects Tanner's quick, deceptive, well-varied serve, but does not fear it.

"I'm just going to stay way back and chip every ball back. I like playing guys with big serves," said the flamboyant New Yorker, who like McEnroe grew up and lives within 20 minutes of the National Tennis Center. "I don't think Roscoe's second serve is that big. If I can get him into a long match, I think he'll be like a pitcher who has to throw too many pitches. I can get to him. He's on a streak, but I feel very confident against him."

So who'll win? Reason says Connors over Gerulaitis in the final Sunday, but I'll pick McEnroe over Tanner. Just a hunch.

If it doesn't work out that way, you can forward my mail care of American Express, Zurich.