The main idea at Wimbledon this year has been to see if anybody could break the seeming lifetime lock on the championship of poor, crippled Bjorn Borg, who demonstrated an inability yesterday to run the 100 in less than 10- flat - in sneakers and moneybelt.

According to the Virtuous Viking, Lennart Bergelin, the all-purpose second, his man, the infirm Borg had about as much chance of getting through his third-round match against Hank Pfister as Camille did of going beyond the third act.

His muscle pulls were worse than hoof-and-mouth disease, and there was "only a 25 percent chance" that Bjorn might last the match. As it happened, the only things critically pulled were the legs of those who listened to Bergelin, Pfister, run down, 6-4, 6-1, 6-3, looked as though Borg had pulled a gun on him. And used it.

Despite Borg's predictable success, this tournament has also been a mish-mash in search of the most unlikely survivor, and he turned up in the left-handed person of Bradley Dara Drewett.

Brad Drewett, a 20-year-old from the Sydney suburb of Killarney Heights, is not to be confused with Rodney Laver or Tony Roche, the most recent Australian lefties to grace the round of 16. But he looks almost as good at the moment to the homeland, in his way a case of beer in the desert. Because Australia has become a tennis disaster area. Dried up. Tapped out.

Once the monarchs of this hydrangea-spattered temple dedicated to the swatting of tennis balls and the swallowing of strawberries and cream, the Australians don't hit tennis balls well enough any more, and they get razzberries in the newspapers at home.

"I am so sick of reading in our papers, 'What's happened to Australian tennis?' and in the second paragraph, 'What's happened to Brad Drewett?'"

The speaker was a slightly giddy Brad Drewett, a burly kid with a heavy shock of blond hair, who remembered the critical stages of his 4-6, 7-6, 7-6, 6-4 victory over Victor Pecci as "a total blur."

He was also self-conscious as three Australian cameramen tried to pose him, first raising one finger in the No. 1 signal, then with two fingers in a V for victory. "I'm choking on my smile, and I can't feel my mouth," he said. Drewett didn't feel like No. 1; but he felt "a lot better than the ranking I came in here with" - 144th in the world on the pro computer.

That was not good enough for acceptance at Wimbledon, and so for the second straight year he went slumming in the qualifying tournament. This time he came through.

"I was into Wimbledon for the first time, although I played here in the junior tournament in 1975 and 1977 when I was the top junior at home. In '75 I was in the semis against Chris Lewis and led him 4-1 in the final set. He came back to beat me, and won the title.

"That gave me nightmares for a while, and it went through my mind today when I led Pecci, 5-1, in the last set and he got back to 5-4 before I closed it out."

Drewett has experienced the pitfalls of the fringe player. He flew to Bombay from Taiwan for a qualifier, and defaulted against a local player over a misinterpretation of the rules. He flew to Cairo to try to qualify there, but was refused entry to the draw.

"They didn't get my cable, they said. I slept on a floor because they had no room for me, either."

He did gain the final at Hialeah, Fla., on the satellite circuit and from there to the edge of the quarterfinals at Wimbledon is an unlikely leap. But it is gratifying to this Australian lefty you won't mistake for Rod Laver and his despairing tennis constituents Down Under. And Drewett did it without a single pulled muscle, and not even a strained groin, or stretched alibi, which is most unusual at the Big W this week.