A shrewd observer of the tennis scene once noted correctly that there have been more "new Jimmy Connors" than there ever were "new Nixons."

And so it is with some skepticism that the savvy have watched the latest "new Jimmy Connors" -- agreeable, relaxed, glowing with the joy of recent fatherhood -- progress through the first three rounds in defense of his U.S. Open championship.

There was a little bit of both the old and the new Connors on display today as "Jimbo" celebrated his 27th birthday by walloping Bruce Manson, a 23-year-old left-hander from Los Angeles, 6-3, 6-0, 6-3.

Connors sounds genuine when he says he is a new man since his wife, Patti McGuire, a former Playmate of the year, gave birth to their first child, Brett David Connors, Aug. 1.

"There's a lot of different things that I do now once I leave the tennis courts. I change diapers and things like that," Connors said when asked if the responsibilities of fatherhood had helped diminish the pressures of his career. "It's a little different than going back and just having to worry about myself."

Then again, Connors has not lost a tennis match as a father. He has won the only two tournaments he has played since getting jolted by Victor Pecci in the semifinals of the French Open in June and by Bjorn Borg in the semifinals at Wimbledon three weeks later. He thinks he, and not the topseeded Borg, will win the U.S. Open.

It was easy being Mr Sweetness and Light as he won his fourth U.S. Clay Court title at Indianapolis last month. It was easy being Mr Maturity as he won an Open tune-up in the beautiful White Mountains of Stowe, Vt., on the same rubberized asphalt surface on which America's premier tournament is being played at the National Tennis Center in Flushing Meadow Park.

But how will Connors react if he does not triumph in the Open, the tournament he has deemed the most important to him after reaching the final five consecutive years, winning in 1974-76-78? How agreeable will he be if his arch-rival Borg, who has whipped him in three meetings on three different surfaces this year, wins instead?

Until those questions are answered, people who have been around can be excused for hesitating before proclaiming Connors father of the year, a born-again hero for America. Past experience suggests that if and when he loses, we might still have the old Jimmy Connors to kick around again.

Connors appeared thrilled as tournament officials presented him a cake at lunchtime, and as the electronic scoreboard in Louis Armstrong Stadium flashed "Happy Birthday" as he warmed up for his match.

He played well and behaved impeccably in the nationally televised match against Manson, a compact, 5-foot-8, 150-pound former All-America from the University of Southern California who is one of the few players to beat Borg this year, having done so in the first round of a tournament in Palm Springs in February.

But after the match, Connors ducked the customary group interview with reporters -- a trick of the old Connors, well-practiced here a year ago and at Wimbledon this summer -- and beat a hasty retreat from the premises, apparently concerned because of a slight muscle pull in his right thigh suffered late in the match.

Connors was never in trouble in his 1-hour 31-minute breeze through the third round. He lost his serve only once, in the third game of the last set, but he ran off the last five games from 1-3 down.

Connors told officials he would attend a routine press conference after getting a rubdown, then changed his mind. As reporters cooled their heels in a room underneath the stadium waiting for him, he signed autographs, posed for snapshots and answered questions about his son with idolators outside the players' lounge.

Then, without showering or getting treatment, he and a couple of his ever-present "corner men" headed to a waiting chauffeured limousine, where his mother was waiting. On the way out, he said he was "all right, nothing wrong."

To those who had sought him out in his brief visit to the dressing room, Connors explained that he had slightly pulled a groin muscle in the next-to-last game of the match.

"It's okay. I just stepped down on it wrong. It's a little sore, but it will be fine tomorrow . . . I'm in good shape. Everything is goind good. I'm playing better and better, nothing but better," he said.

"I hit the ball good today. I hit the ball solid and I moved good. I'm just glad I got to play today. I already had two days off," he added, noting that he got on the rain-delayed afternoon program only because Englighman John Lloyd defaulted his match against third-seeded John McEnroe. Lloyd, husband of Chirs Evert, was suffering from stomach pains, apparently the after-effects of heat exhaustion following his four-hour, 5-7, 6-7, 7-5, 7-6, 7-6 victory over Paul McNamee in the second round.

Connors looks lean and mean on the court. His weight is down to 147 pounds, the lightest he has been since his world-beating year, 1974, when he swept the Australian, Wimbledon and U.S. Open titles and lost only four matches.

"I'm also a little lighter mentally than I have been for the past couple of months," he said after his first-round victory over Anand Amritraj, pooh-poohing the significance of his losses at Paris and Wimbledon while stopping short of blaming them on the anxieties of impending fatherhood.

"That is more important. I'd rather play with extra weight on my body than extra weight on my mind. It so happens that I've come here in pretty good shape both ways, physically and mentally. I've had some real good practice over the last few weeks and some real good matches. They got me in pretty good shape.

"I got something better than slumps, I'll tell you that, and that will only make me play better. My wife's okay. My baby's okay. So that means i'm okay. And everybody else should be okay."

In other matches, Chris Evert seeking her fifth consecutive Open title, drubbed Renee Richards, 6-2, 6-1.

Pat DuPre, the surprise Wimbledon semifinalist this year, trailed 0-3 and two service breaks in the first set but regrouped his thoughts during the rain delay and came out playing a much more controlled game to beat 13th-seeded Gene Mayer, 7-5, 6-2, 6-3.

Brian Gottfried, the No. 11 seed who will meet Connors next, survived a scare from Victor Amaya, the 6-foot-7 "Incredible Hulk" with the mammoth left-handed serve, 6-7, 4-6, 7-5, 6-1, 6-4; Wimbledon runner-up Roscoe Tanner beat Nick Saviano, 6-4, 6-4, 3-6, 6-3.

Australian Dianne Fromholtz, seeded No. 6, beat Leslie Allen, perhaps the most promising young black woman player in the U.S. 6-7, 6-3, 6-2, after Allen came back from 0-5 in the first set; No. 3 seed Tracy Austin blasted fellow California Kate Latham, 6-3, 6-0, while veteran Kerry Reid, seeded eighth, barely survived a challenge from Mary Lou Piatek, the Wimbledon junior girls' champion, 4-6, 6-3, 6-4.