The latest wunderkind of tennis, John McEnroe, was practicing on an out-of-the-way court yesterday, sending one low, wicked shot after another across the net, and a player his age but not nearly his equal murmured: "What a marvelous gift."

That also often is the closeup assessment of McEnroe. At 18, he has an adult game, and much of tennis - surprisely - is surprised that he does not also always have adult manners. In the two weeks since becoming the youngest semifinalist at Wimbledon, McEnroe has discovered the sporting life to be anything but one large bowl of straw-berries and cream.

The attention of late has been grand - at times. "I was hoping it would be like this," he said. "People noticing me, not just because I'm 6 feet tall but because I must be doing something fairly well. I don't go out of my way to seek autograph situations, but that does come with winning.

"Last year, I was playing juniors, and maybe I signed 10 autographs all year. Near it's more and more every day." Often it is like yesterday, a tot tugging at his warmup jacket as he was about to buy a Cracker Jack. Or reporters interrupting a meal. Or fans that expect him to win every match. Or oficials.

"They (assorted linesmen and umpires) are looking for me," he said. "They're out to show they can straighten out this 18-year-old kid." Apparently, what played in Wimbledon, or at least was tolerated there, did not play last week in Cincinnati.

"The crowd was not for me." McEnroe said. "I didn't say one word the last two matches (he lost in the quarterfinals) and still they weren't for me. The papers said I was warned after yelling about a few calls. That really wasn't it. They (the officials) said I was out 65 seconds instead of 60 between changes of serve.

"So it's tough, you know? I don't want to be known as a bad boy. I don't think I have a bad reputation, and I don't want to get it."

Washington fans will offer their judgement today, when McEnroe meets the local favorite, Harold Solomon, master of the moon ball, in a first-round match of the Star tournament.

"It's not a great draw for me," he said, "but then I've gotten my share of breaks that way." He was referring to his having to face such as Ismail el Shafei, Colin Dowdeswell, Karl Meiler, Alex Mayer and Phil Dent at Wimbledon before meeting Jimmy Connors in the semifinals.

"That one, with Connors, took a change in attitude," he said. "Instead of a gee-it's-great-just-to-be-playing-someone-that-good feeling, I tired to convince myself I had a chance to win."

He lost in four sets - and then began to learn a bit more about tennis and his place in it.

"So much of this is mental," he said. "That's the key. I played the week after Wimbledon, when I was done in, and I won one match and then lost." And then apparently made a firm decision not to turn pro - "apparently" because his father is said to have pushed exceedingly hard for college.

"It'll be Stanford, probably for at least a year," he said. "This might be a little early to determine exactly what I want to do with my life. I'll see what college is like.

"No, I don't know what I'll major in. Something as easy as possible, I guess. But then I wouldn't be going to Stanford if I wanted the easy way out, would I? But I do have to be pushed." Athletically as well as academically, if often seems.

?I just can't play in practice," he said. "I'm the worst practice player there is. But now I've got to do more, stay in shape. But not overdo it, keep from getting tired mentally. Before Wimbledon, I kidded people by saying I'd turn pro if I won. And then I got to the semis."

And then the sporting spotlight began to get intense. Where once McEnroe won in relative obscurity, he suddenly attracts national headlines when he loses. And he keeps getting public spankings for often acting his age, for griping so often he does not get the benefit of doubt on questionable calls.

Usually, there has been at least an occasional athletic distraction for McEnroe, especially soccer and baskeball. Perhaps there also will be at Stanford. Perhaps not. The grip of tennis may become more intense.

"It would be nice to do more than one thing," he said. "Like Vilas. He's a poet, and he went to law school. That's to his advantage." For the moment, it seems to McEnroe's advantage not to grow up in public.