Maybe it was his back.

A doctor said Jimmy Connors had a bad back the other night when he was beaten by John McEnroe in the semifninals of the U.S. Open tennis tournament. Connors clutched at his back in apparent pain and abstained from his usual street fighter's swagger.

As the decisiveness of his pending defeat became clear, Connors grew more sedate. The correlation moved a cynic to say, "Jimbo can get a bad back any time he needs it." The things some people say.

Back or psyche, Connors' pain won't soon go away.

The Open result, with McEnroe winning the championship, certifies a change at the very highest levels of tennis. For the first time in six years, it can be said that Jimmy Connors is not the No. 1 or No. 2 player in the world. He likely is third best, behind Bjorn Borg and McEnroe, and a case can be made that Roscoe Tanner on the rise, has pushed Connors to No. 4.

Strange, the idea of Connors as No. 4.

Impossible, the idea of Chris Everet as No. 3.

Yet the defeat of Evert by Tracy Austin in the Open final is signal she is no longer our queen unchallenged. Martina Navratilova is No. 1 on the computer, with Everet second, but both were beaten in the Open by Austin, who at 16 and in her first year as a professional is figuratively decades younger than Evert.

Athletes reach their physical peaks at 28. Connors is 27, Evert 24. Their decline, clearly visible though certainly not precipitous, is mental. The flame of single-mindedness that once burned fiercely in them is a nostalgic glow. They have moved away from the blast furnace of desire to a cozy spot by the fireplace of middle age contentment. Both are recently married, Connors the father of a month-old child and Evert eager to trade her rackets for diapers.

At 20, McEnroe is a master of his craft, who in the moments after his straight-set rout of Connors was bold enough to say Jimbo really ought to work on his game.

"If I was him," McEnroe said of Connors, who in the previous five years won the Open three times and finished second the other two tries, "I'd try to improve on my serve

"Especially against Borg, when you serve and stay back, Jimmy's serve is not going to do anything. Borg is just going to top it back deep and he's already in the point.

"Jimmy's gotta try to get that serve as a weapon. And it's not a weapon right now. Another thing is, I'm sure Jimmy is a great player, so he must be capable of volleying well if he decided to. Against Borg, and against a few guys now in the touch matches, he's gotta attack more . . . why not change his game a little and prove he is a better all-around player. People would talk about his greatness more, that he was able to be versatile."

As it happens, someone had asked Connors earlier in the week if ever he might change his game.

"Why change a good thing?" Connors said. "It's been a good thing to me for 25 years . . . If I got to change that because of one guy or two guys or three guys or 200 guys, get me another job."

For the first time in six year, Connors has not reached the championship match in any of the world's three most important tournaments -- the French Open, Wimbledon and the U.S. He lost in the semifinals each time, first to Victor Pecci, then Borg and McEnroe.

McEnroe, in addition to his Open victory over Connors, also owns a 1979 victory over Borg, having defeated the Swede in the final of the World Championship Tennis Playoffs. McEnroe has won 12 tournaments this year to Connors' nine. "I consider myself, on a given day, the best." McEnroe said. "But I think Bjorn is the No. 1 player in the world for the year. I'm either 2 or 3, depending on judgment."

McEnroe had Connors in mind in the debate over who's No. 2, but Roscoe Tanner ought to be mentioned.

Tanner made it to the Wimbledon final, where he lost to Borg in five sets; he defeated Borg (which Connors has not done this year) in the U.S. quarterfinals; he won at Palm Springs with both Borg and Connors in the field, and has had his most consistent year yet, never losing before the quarterfinals.

Like Connors, Tanner is 27.

Unlike Connors, Tanner has never won a major championship.

"If I didn't think I was getting better," Tanner said, "something would be wrong."

Connors, as we heard, is content with his game. And why not? It has won everything. Connors' only problem is the same as Evert's, which she described memorably soon after Austin ended her four-year winning streak in the Open.

"Tracey's only 16 and if her nerves don't get her, one day, next year or the year after, she will be No. 1," Evert said.

Nerves?

"It's easier winning it the first time. I'm definitely not as hungry as I used to be. At 18, I was very hungry, very determined, I was after every title.When you're that young, you've got nothing to lose. So Tracy went for all her shots because she was expected to lose.She probably felt, 'If I don't win this year, I've got 10 more years to do it.'"

At 24, having won everything several times over, Evert said the pressure of expectation is great.

"It's tougher being on top with everybody out to beat you. It's much tougher to walk on the court feeling that way every single time. I feel much more pressure now than when I was 18. I feel more emotion, whether it's panic or what.Years ago, whenever I would play Billie Jean King, I was very calm because I was expected to lose."

In defeat, Evert was given a three-minute standing ovation by a stadium crowd of nearly 20,000. Applause for Austin lasted maybe 20 seconds. In that contrast is proof of Evert's everlasting fame and Austin's shining promise. And Evert's contentment in defeat contrasted markedly with Austin's feistiness in victory. That feistiness was made clear when someone suggested to Austin that perhaps Evert didn't play all that well in the final.

"Well, I think I played really well," Austin said snippily, adding, "like, it seems, that every time I beat Martina, 'Martina has a cold,' or 'Martina just flew in from Japan,' or 'Martina broker her arm.' . . . I played really well today. Give me a little credit."

Austin now has beaten Evert three of five times in 1979. If Evert the queen is a doddering 24, she knows that Austin's time will come "Somebody now will come along like Andrea Jaeger (who is 14)," Evert said. "In three or four years, Andrea could be challenging Tracey."

Then we will learn about Asutin's nerves,