If Bjorn Borg now wins the U.S. Open tennis championship this weekend, and goes on to complete the Grand Slam in Australia at the end of the year, he may well look back on his gritty 6-4, 3-6, 4-6, 7-5, 6-3 quarterfinal victory over Roscoe Tanner today as his most harrowing match: another testing moment of truth for an extraordinary champion.

Their 2-hour, 53-minute duel in the midday sun didn't have the majesty, the sustained high quality, or the excruciating intensity of Borg's all-time victory over John McEnroe at Wimbledon in July, which was a rare masterpiece of art, drama, heart and soul, truly a tennis match for the gods.

This one was only semi-excruciating. The tennis was spotty. But in fighting back from two-sets-to-one behind and 2-4 down in the fourth set against a fiercely determined opponent on a day when he wasn't anywhere near his best, in suddenly elevating the level of his play from the ridiculous to the sublime at just the moment when he had to do so or face defeat, Borg again proved he is a remarkable competitor.

The Swedes have been telling us since Borg was 15 that he has "is i magen" -- "ice in the stomach" -- and he keeps proving it under fire.Astonishingly -- as he reminded the media afterward in his quiet, unassuming, modest but immensely proud manner -- he has not lost a match that went five sets since 1976.

It was Tanner who conquered Borg in the Open after extending him to five sets in the Wimbledon final last year, and for awhile it appeared lightning would strike again out of a cloudless blue sky. This was an idyllic Indian summer day in Flushing Meadow -- heat without humidity -- but there was lightning in Tanner's electric serve.

Borg looked vulnerable, and excited murmurs of impending upset rippled through the sunbaked crowd of 17,381. But Borg dug his way out of doom and will play South African Johan Kriek -- who came back from 2-5 down in a final set tie-breaker and beat Poland's Wojtek Fibak, 4-6, 6-2, 3-6, 6-1, 7-6, in a dazzling four-hour quarterfinal tonight -- for a place in Sunday's final.

Kriek won the last five points to take the tie-breaker, 7-5, after Fibak had climbed back from 1-4 down in the final set of an animated, rousing match that enthralled the evening crowd of 18,187. Though Borg-Tanner will be recalled years longer, this was blood-and-guts tennis, too, as both players struggled to get to the semifinals of a grand Slam tournament for the first time. It was full of acrobatic net play, diving "gets", uncompromising rallies, and 18 service breaks. The game that got the swift and flashy Kriek to 4-1 in the fifth set lasted 30 points and went to deuce 12 times.

For long stretches, this afternoon's match resembled last year's quarterfinal in which Tanner ambushed Borg under the lights here at the National Tennis Center, putting a premature end to his dreams of a first Open title and a possible French-Wimbledon-U.S.-Australian Grand Slam.

Once again, Tanner was serving lefthanded blurs : 19 aces, 26 more service winners. Once again, he was relentlessly attacking Borg's second serves, hacking or chipping them deep and boring to the net behind his returns to challenge the Swede with penetrating volleys.

Once again, Borg was clearly troubled by Tanner's aggressive tactics. He wasn't returning well -- not only the first serve that he was lucky to get a racket on, but the second serves as well. He was frequently mis-hitting and floating them short, permitting Tanner to come in on every second serve and usually get a good crack at the volley.

And once again, Borg looked peculiarly sluggish and lethargic. He wasn't serving well. After a good start, his first-serve accuracy plummeted to 53 percent in the third set and 54 percent in the fourth. He was making uncharacteristic unforced errors. At his nadir, late in the third set and through six games of the fourth, he wasn't moving his feet. He looked hot, tired, beaten. His expression bore the troubled, vacant look of a loser.

Borg missed five of six first serves and double-faulted twice in the first game of the fourth set, but held serve to 30. Tanner missed four of five serves but held to 15 as a loud voice screamed from among the crowd of 17,381: "Don't let up, Roscoe!"

So it went. Borg missed four of eight first serves, but held to deuce for 2-1 Tanner missed five of six, but Borg wasn't putting any pressure on him with his returns, and won only one point as Tanner held for 2-2.

When Borg lost his serve to 15 in the fifth game -- butchering a forehand groundstroke and a forehand volley to 0-40, then knocking a late and hurried backhand wide off a rocket Tanner return of a second serve -- he seemed finished.

"We're seeing history," said a man in the press box, ruefully making plans to cancel his reservation to Melbourne to watch Borg, the five-time French Open and Wimbledon champion, go for an Australian Open crown and the Grand Slam 10 days after Christmas.

But history unfolds in curious ways. At that moment, when Borg's face suggested he was on the despondent side of impassive, he wasn't thinking gloomy thoughts. He was not about to surrender. He was, he said later, recalling that in last year's defeat he came back from 2-5 down in the foruth set before losing it and the match in a tie-breaker.

"I just remembered that match, and thought if I could just hold my serve for 3-4, to stay within one break of him, maybe he wouldn't serve so well under the pressure," Borg said. "I just wanted to put more pressure on him, especially on the second serve, and make him hit a few more balls so maybe he'd start making errors. I did it, and that's exactly what happened."

Relaxed and loose as only he is when behind and in such a perilous position, Borg played his best service game of the set, holding to 15. Now it was 4-3, Tanner.

Up on his toes, his close-set, glacial eyes riveted on the ball, Borg walloped a forehand passing shot: 0-15. Then Tanner served a bullet that he thought was a fault, but it wasn't called, and Borg ripped a backhand return that resulted in Tanner hitting a forehand volley into the net: 0-30.

Here Borg floated a backhand return that looked like something from earlier in the set, but Tanner did not crunch away his first volley, and Borg hoisted a high, lazy backhand lob from the baseline. Tanner backpedaled, but couldn't kill the smash, either, and Borg scythed a backhand cross-court that fizzed with topspin, inducing a stretching volley error: 0-40.

Tanner saved one break point with a superb drop volley, then missed another first serve. Borg swayed from one muscular leg to the other as he awaited the second ball. Whack, he drove a forehand return down-the-line for a winner. It was 4-all. The audience erupted.

After that, Borg was his old self -- if any fellow of 24, even a certified immortal, can have an "old" self. The verve came back to his game. He ran like the wind, made some astounding shots, and let his peerless instincts and court presence take over.

Not that it was easy the rest of the way. Far from it. Borg let Tanner escape two set points from 15-40 as he served at 4-5, reviving doubts. Maybe he would lose in a tie breaker once again, the last ironic reprise of last year's match.

But at 5-6, Borg broke at love for the set.

Borg netted a backhand volley off a rat-a-tat-tat exchange of four shots in the forecourt to fall behind 15-40 in the first game -- "a very, very important game," he called it later -- but saved one break point with an incredible running forehand cross-court passing shot, and another as Tanner steered a forehand wide.

Borg was off on an amazing spree that was reminiscent of the fifth set against McEnroe at Wimbledon, where he escaped a 0-30 hole in the first game and lost only one more point on his serve in the match. Today, from 15-40 in the opening game of the final set, Borg ran 19 consecutive points on his serve not conceding another until he was serving at 5-3, 40-0 -- triple match point.

Tanner kept fighting gamely -- holding serve after double-faulting to break point in the second game of the final set, holding to 15 with an ace for 2-2, and to love with a service winner for 3-3.

But Tanner was starting to get cramps in his upper thigh, and his legs were stiff. The spirit was willing, but the flesh was too weak to keep up with the resurgent Borg, who was now running around Tanner's second serves and banging forehand returns, making things happen.

Serving at 3-4, Tanner saved three break points and got to advantage, then double-faulted. He was limping noticeably.

Borg climbed on a second serve and lashed a forehand cross-court return winner: break point No. 4. Tanner got in his first serve, but Borg jerked him out of position with a forehand cross-court, made him stretch for a backhand angled volley, and swooped across the court to hammer a forehand pass down-the-line. Tanner wanted desperately to chase the ball, but it was already behind him. He pulled up short -- and broken.

"A couple of times in the fifth set, I hit deep volleys to his forehand corner that I thought were winners, but just in case, I covered the down-the-line passing shot. He almost always goes that way. This time he not only got to the ball, but hit cross-court passing shots on the run. That's a tough shot, and I had never seen him hit it in all the times I've played him," Tanner said admiringly.

"That's just too good. I thought I should have closed out the match from 4-2 in the fourth, but I didn't lose it. He won it."