They get in your head, shake your nerves, turn your feet to putty. They're the ones who toss up double-digit ace days and render useless quick feet, quick hands, quick minds. Boom. They are the big servers of tennis, such as 19-year-old Pete Sampras, who rode his cannon of a serve to a U.S. Open championship on Sunday.

They are the Nolan Ryans of their trade. Rod Laver wasn't called "Rocket" for nothing. John Newcombe could bring it. Roscoe Tanner felled a few opponents with a master blaster or two. Recall William Tatum Tilden and Robert Falkenburg. Nowadays, David Pate, Sampras, Boris Becker and Goran Ivanisevic have four of the bigger serves in the game.

"It's all in timing and rhythm," said Tanner, who was timed as high as 153 mph in his best days in the late '70s. "Pete's not a big guy {6 feet} either, just like I'm not a big guy {also 6 feet}. We're not big on lifting weights. The toss is placed where he can hit it, where his racket naturally meets it. A lot of people throw the ball up, and then go after the ball."

A good serve is not synonymous with velocity. Far from it. John McEnroe had the tangiest, most wicked serve you ever did see, and he may have struck the ball just a little harder than Michael Chang does today.

"You use two or three criteria," said Arthur Ashe, who could mash in excess of 110 mph on his good days. "The first is, how good is your second serve? Second, how deep is it? Third, can you put it where you want to put it? McEnroe, when he was good, had a great second serve. You just did not know where it was going. To me, his serve was akin to Juan Marichal. He turned his back to you and you had no idea what to expect."

Ashe pronounced Sampras "just awesome. I hate to be trite, but gee whiz. . . . He has extremely long arms. His wingspan of body to arm length is one of the longest on the tour. And he doesn't serve that hard. The ball just zooms out of there. He's obviously playing with a very stiff frame. I would shudder to think what he would do with one of those wide-body frames. He just has a standard graphite frame."

Sampras had 13 aces in his three-set championship match against Andre Agassi, 17 in his four-set semifinal against McEnroe, 24 in his five-set quarterfinal against Ivan Lendl. When his serve is on, it wins the point for him; Sunday, he won 92 percent of the points after he got in his first serve.

Sampras served 100 aces during the fortnight. Riding a big racket, a guy can do a lot of damage, as Tanner did in the 1979 Wimbledon, taking Bjorn Borg to five sets in the championship match. That means almost no margin of error in holding your serve. And you, after all, have only the normal, non-ballistic type.

"You never get a rest," Ashe said. "What it also means is when the match is over, Sampras is going to have a lot of cheap points. You serve and the point's over. Somebody like Chang, he's just not going to ace you."

Tanner thinks Sampras is probably faster than his advertised 120 or so mph.

"They time the ball differently now," he said. "He must be faster than 124. The mechanism is timing the ball as it's passing the net or hitting the net cord. Mine was timed as it hit the racket. I've been in some big-serve contests as a judge, and they were hitting 125 in a mall in regular clothes."

Pate's serve has been measured as the fastest this year -- 125 mph -- but Sampras has two of the next four fastest. There are aerodynamic reasons and torque and resistance to take into consideration. And there's the layman fact that Sampras hits the yellow out of the ball.

"When I serve as well as I did this week," Sampras said, "you put it in the minds of my opponents that if they play one bad game, the set may be over."