Tom Watson fired five birdies over a seven-hole stretch of the back nine today to race past Lee Trevino and take a four-stroke lead into the final round of the 109th British Open golf championship.

His amazing 64, seven under par for this 6,926-yard course and the lowest round he ever has played in a major championship, gave Watson a three-day total of 202 and a firm grip on first place.

Trevino and his playing partner, 23-year-old Scot Kenny Brown, a Ryder Cup player for Britian and twice a World Cup competitor for Scotland, are four shots back. Trevino, who led by three shots after the second round, bogeyed the last three holes to end the day at par 71. Bogeys at 16 and 18 dropped Brown to 68.

The only other player besides the top three with a reasonable chance of victory is Ben Crenshaw, who also had a 68 today for a 208 total, six shots behind.

A triple-bogey 7 hurt Gil Morgan. Six under par before his misfortune, he finished with 71-211. Putting prove the Waterloo of Jack Nicklaus and Jerry Pate. Nicklaus also had a 71 for 211 and Pate skied to a 74 for a 212.

Incredibly, Watson's 64 was not the lowest score of the day as the weather conditions proved ideal. Japanese star Isao Aoki, who hounded Nicklaus in this year's U.S. Open at Baltrusrol, broke the one-day-old Muirfield record with a 63 and tied the Open record set in 1977 at Turnberry by Nark Hayes. Ironically, the man who shot the Friday 64, Argentinian Horacio Carbonetti, had his second 78 today and missed the cut of low 60 and ties for the final round, as did Gary Player.

"It was an ideal day for scoring," admitted Watson, whose 64 also was matched by Herbert Green. "Maybe it's the calm before the storm." He explained the low scoring over this much-respected British Open venue by saying, "The wind makes the tournament tough. If you don't have the wind it plays relatively easy. If we had Monday's weather the scores might have been in the 80s."

Easy conditions or not, it was an incredible display of shotmaking by the 30-year-old redhead, who won this tournament in 1975 and 1977. What was most spectacular was the fact that while he started making his run at the 11th hole, the pair behind him, Trevino and Brown, were starting pyrotechnics themselves. For better than half an hour it was unbelievable shot answered by unbelievable shot.

Moments after Watson closed within one shot of Trevino by holing out a two-footer set up by a wedge shot on the 11th, the second-round leader came close to holing out a long putt from the 10th fringe, saving par. Brown, who was bunkered on the hole, had just blasted stone dead to the pin for his par.

Then Watson made a 12-footer for birdie on the 12th. It came moments after Trevino had gone up by two again by hitting a seven-iron shot to two feet of the hole on 11 and sinking this putt. Undaunted, Brown, a 6-foot, 140-pound reed called "the walking one-iron" by fellow pro Sandy Lyle, also birdied, from four feet.

Trevino and Brown, the Mutt and Jeff of the fairways today, each parred 12 and heard the roar ahead as Watson pulled even on the 153-yard 13th hole, just catching the hole on the last turn of the ball of his eight-foot putt.

Brown, two strokes back at this point, then hit probably the greatest shot of the tournament to save par at the 13th.

"Stood there and applauded him in the middle of the green," said Trevino. Brown's tee shot had come to rest in extremely deep grass just over a bunker left of the green. His sand wedge shot nearly hit the pin, leaving him a two-footer. "He even put a spin on the ball," said an admiring Trevino.

All three players parred No. 14. Then Watson took a one-stroke lead over Trevino and two-stroke advantage over Brown by hitting his approach shot to within 15 feet of the pin and sinking for a birdie on the 396-yard 15th. This was immediately followed by matching birdies by Trevino, from 20 feet, and Brown from 15 feet.

This epic duel could not continue. Watson just missed his eagle on 17, making a two-foot birdie on the 542-yard par-5, then saved his par on 18 with a four-footer after nearly holing out a bunker shot.

Disaster made company with the day's last twosome.

Trevino bogeyed the par-3 16th after his three-iron tee shot rolled into a bunker. He bogeyed 17 when "the wind switched on me" and he hit his drive into a bunker, was forced to blast into the fairway and three-putted from the fringe. He added a third straight bogey on 18 when he also three-putted from the fringe after driving into the left rough between bunkers. t

Brown fared a little better. He also bogeyed the 16th from a close lie near the greenside bunker. Then he had a 50-foot eagle putt lip the cup on the 17th before making his fifth birdie of the day.

He now looks back at No. 18 as a what-might-have-been hole. He drove into the thick rough on the right side. He elected to play safe with a nine-iron and came up well short of the green from where he got down in three for a closing bogey. "I think I could have slashed a wood at it," he said of the precarious lie following his drive.

At any rate, Brown continues to downplay his chances to win this tournament and become only the second British player since Max Faulkner in 1951 to garner one for the queen. Tony Jacklin did it in 1969.

Saying he was "stunned" Brown commented, "It's unusual for a bad player to do so well . . . generally play badly. Today I played quite well, really."

Aoki, who made the cut by a shot with his pair of 74s, needed only 24 putts for his 62 -- carding a remarkable nine 3s and like number of 4s. Incredibly, he missed back-to-back six-footers which, when he is hot, are lige one-footers to most putting mortals. The first, on the third hole, lipped the cup.

His birdie putts came at the second hole, from three feet; the fifth from 15 feet; the eighth, from seven feet; the ninth, from 10 feet; the 11th, from 20 feet; the 12th, from 30 feet; the 14th, from four feet, and the 17th, from three feet. That last birdie evoked a "making-it-a-mockery" cry from an elderly observer.

Only once did Aoki need a putt to save par. He hit a "heavy four-iron" approach to the 18th green and landed in the clipped grass of the same left bunker that cost Nicklaus a bogey Friday. Aoki's pitching-wedge escape nearly hit the pin, but rolled 11 feet past. He then put another one dead center.

Aoki was unaware the 63 was a record. His interpreter said Aoki "failed to qualify at Turnberry, so he didn't know" what Hayes had shot. When he found out that a 62 would have given him the tournament record alone, as well as another $50,000 from the same golf magazine that awarded Nicklaus and himself that amount for breaking the four-day scoring mark in this year's U.S. Open, the turtleneck sweater-clad Aoki scowled and said, without interpreter, "putt on No. 4 -- miss."