Jack Nicklaus once said, after taking the U.S. Open lead after one round: "Shooting one good round is not that hard.The real test is shooting four of them."

Tonight, Jim Thorpe is thinking just that. Playing early today, with the greens on Merion Golf Club's tight East Course still soft and the galleries small, Thorpe shot the round of his life, a 66, to put himself in the lead after the first day of the Open.

Thorpe, who lived in Falls Church, Va., until a little more than two years ago, birdied the last two holes, rolling in a twisting 30-footer at the 18th to put his 66 on the board at 11:45 a.m., leaving 141 players in the 156-man field with a shot at bettering him.

"I don't think that will hold up for the lead," he said. "The course isn't playing that tough, and there's a lot of good players out there."

The good players came and went and the 66 stood up, making Thorpe the first black to lead the Open since John Shippen was in a six-way tie for the lead after one round in 1896.

One shot behind Thorpe is J. C. Snead, the 1978 runner-up. Snead, wielding his one-iron off virtually every tee, chipped in from the fringe with an eight-iron at 18 for his final birdie.

Behind Thorpe and Snead, the field was bunched. Five players were at 68, including 1979 PGA champion David Graham, John Cook, John Renner, Chi Chi Rodriguez and club pro Bob Ackerman, a man who gave up trying for his PGA tour card after failing seven times and who played in the first group off the tee at 7 a.m.

Jack Nicklaus, the defending champion, and past champions Johnny Miller and Hubert Green were in a large group at 69, including former University of Maryland golf captain George Burns. Tom Watson, seeking his first Open title, shot 70, as did Bob Crenshaw, Bruce Lietzke, Bill Rogers and 1976 champion Jerry Pate.

Lee Trevino, who won here 10 years ago, shot 72. Lee Elder of Washington, D.C., also shot 72.

Thorpe, 32, mindful of the single-day flashes of the past, was pleased but hardly exultant over his round on the 6,544-yard course. "I wedged it well and I putted it well," he said. "This is a good course for me because I don't have to hit my woods that often. I hit them long but if I get two out of five in the fairway, I'm doing well. On this course, I can hit a lot of irons. Today, I kept the ball in the short grass, so I was able to score."

Snead isn't leading because Thorpe made a mockery of the legend that holds that par-par on the last two holes at Merion is nothing short of brilliant. He was two under par when he arrived at the 17th tee.

That hole is a brutish, 224-yard par-3 played over the rock quarry that cuts across the last three holes of the course. Late in the day, Watson came to the hole one under par and barely escaped with a bogey.

Thorpe, playing in the Open for the first time, was not intimidated. He pulled a three-iron from his bag and slammed the ball within three feet of the pin. The birdie was a virtual tap-in.

At 18, Thorpe hit what may have been the longest drive of the day, down the left side of the fairway. From there, he hit a five-iron (most players use about three-iron) to the front of the green, 30 feet short of the pin.

Thorpe, thinking two-putt, did much better. His twisting putt jerked left at the final moment and into the hole for a birdie.

As the day wore on, one challenger after another made his run, then fell back. Renner was four under through 14 before finding a bunker at 15 and ending up with bogey. He then three-putted 16 and went on to 68.

Renner's problems didn't come close to John Schroeder's however. Noted mainly for being the slowest player on the tour, Schroeder was four under through 12 today. He then proceeded to play the last six holes in five over for a 71.

Nicklaus was two under at one point and his 69 was unspectacular but steady. He bogeyed the third hole but rolled in a 20-foot putt at No. 8 for par. Birdies at 10 and 11 got him to two under before he was bunkered at 15 for his final bogey of the day.

"I started kind of ragged but I played a pretty good round of golf," Nicklaus said. "Anytime you break par in the first round of the U.S. Open, you've played well. I'm quite satisfied."

"The course is going to get faster and tougher." Watson said. "A 66 the next couple of days will mean more than a 66 did today. There are a lot of guys at 69 and 70 and at least one of them will shoot a very good score tomorrow."

Watson's message, in different words, was essentially the same as Nicklaus'. All agreed Thorpe's 66 was impressive. All wanted to see what he will shoot Friday.

"I'm just glad to play a good round because I've been playing so poorly all year," said Thorpe, who is 146th on this year's money list with $7,146 in winnings. "The important thing today was that I putted so well. If I can putt like that the rest of the week. . ."

Clearly, this tournament is still wide open. Players like Greg Norman (71), Craig Stadler (71), Isao Aoki (72), Gary Player (72), Hale Irwin (72) and Severiano Ballesteros (73) must still be considered contenders.

Those with more remote chances include people like second-leading money-winner Raymond Floyd, who had 75; 1975 champion Lou Graham, who had 76, and 51-year-old Arnold Palmer, who had 77.

Other than Thorpe, the most surprising contender is Ackerman, an assistant pro at the Detroit Country Club. "I was lucky because I played my practice round with Palmer yesterday," he said. "I was so nervous then I could barely get the tee in the ground on No. 1. But I got rid of most of my first-round jitters then, and today, I was just fine. I'll take four rounds like this and be delighted."

Thorpe felt the same way.