Ilie Nastase declared on the eve of the Wimbledon tennis championships that he would not shave until he was beaten. He offered no rationale for this decision - logic in such a case would be distinctly unNastasean - but somewhere in the tangle of his superstitious psyche he sensed that this is the year for stubble.

As stately Wimbledon got down to its nerve-testing second week and business of sorting out the serious contenders from the pretenders to the throne yesterday.Natase's handsome countenance bore the look of a Skid Row bum. But his game was an immaculately groomed as the hedges and rosebeds of the All England Lawn Tennis and Croquet Club.

He played instinctively masterful tennis in eliminationg sixth-seeded Roscoe Tanner, 2-6, 6-4, 6-3, to earn a place in the quaterfinals for the fourth time.

"Nasty" displayed the rich gifts of his genius without it s tiresome, unpleasant eccentricites. He needed no hand-outs, but got one in the form of a dubious double-fault call against Tanner on the fourth match point.*

Nastase, runner-up in 1972 and 1976 and keen to make another run at age 31 for the title he yearns for most, served superbly. That was the single most important factor in his victory.

But it was his breathtaking quickness, his feel for the ball and control of it in chilling, swirling wind that captivated 14,000 Centre Court spectators, most of whom were bundled up in overcoats, some in mufflers, and gloves.

Nastase had to play beautifully to steal the day's attention from last year's finalist. Bjorn Borg and Jimmy Connors, who found the form that most observers thind will carry them to a rematch of last year's five-set slugfest for the crown.

Borg, seeking at age 22 to become the first man to win three successive singles titles since Fred Perry in 1934-35-36, suffered only a couple of brief lapses in mauling Australian Geoff Masters, 6-2, 6-2, 6-4.

Connors looked as ruthlessly lean and hungry as he did in 1974, his title-winning year. Bashing the ball and boring relentlessly in to the net, he made only two unforced forehand errors in the first two sets and never had a break point against him until he led by two sets and 3-1 in the third.

It was another difficult day in one of the most testing of Wimbledon's 101 years. The wind was gusty and unredictable, with a winter bite, and melancholy gray mountains of clouds filled the sky.

Play again started at noon, two hours earlier than usual, as the referee tried to unclog a program log jammed by rain. The entire men's and women's singles fourth round were played, but more rain late in the day further snarled the doubles schedule.

"It's hard to tell who is really sharp and who isn't, because on a day like this you have to play the condition more more than anything else," said Billie Jean King, who took an injection of novacaine to ease the pain of a bone spur in her left heel and then blasted Sue Barker off the windblown Center Court, 6-2, 6-2.

Brian Gottfried, the No. 5 seed, returned serve manificenlty in erasing Tim Gullikson, 6-2, 6-4, 6-2, on court No. 2, which was so torn up that Gottfried noted, "It looked like it had gone through a war."

Tom Okker similarly served and volleyed industriously, and hit enough winning returns and passing shots to defuse the cannonading serves of California Tom Leonard, 6-1, 9-8, 6-1, out on court No. 9, which wasn't much better.

Raul Ramerez, the only man who had reached the quarters without losing a set, made some telling forehands and acobatic volleys in silencing the last roar of old lion John Newcombe, the champion of 1967-70-71, 6-2, 9-8, 6-3.

Vitas Gerulaitis, after double-faulting twice in the game that cost him his second set, got his flashing attack together and blanketed the net to oust dangerous Hand Pfister, 6-3, 3-6, 6-2, 6-3.

Martina Navratilova overwhelmed 15-year-old Tracy Austin, 6-2, 6-3; Chris Evert methodically scythed down troublesome Kerry Reid, 6-3, 6-4; and Evonne Goolagong encountered virtually no resistence from Regina Marskova, 6-2, 6-1.

Defending champion Virginia Wade battled her concentration and struggled to a 7-5, 6-4 victory over Ruta Gerulaitis.

There were three minor upsets in the women's singles.

Virginia Ruzici of Rumania, who won the German and French Opens over fields depleted of top women by World Team Tennis, ambushed last year's runner-up, Betty Stove, 5-7, 6-4, 6-3; Mima Jausovec of Yugoslavia toppled Australian Wendy Turnbull, runner-up at last year's U.S. Open, 3-6, 4-6, 6-3, and South African Marise Kruger startled Australian Dianne Fromholtz, 4-6, 6-4, 6-3.

At the end of the day, the quarterfinal pairings emerged like this:

Men - Borg vs. Sandy Mayer (A 6-4, 6-2, 6-8, 1-6, 7-5 Victor over Wojtek Tibak). Okker vs. Nastase, Gottfried vs. Gerlaitis and Connors vs. Ramirez.

Women - Evert vs. King, Wade vs. Jausover, Goolagong vs. Ruzici, and Kruger vs. Navratilova.

Nastase produced the most fascinationg shotmaking in sidetracking a curiously flat Tanner, whose usually electric serve was short-circuited into far too many faults.

Player-coach of the Los Angeles Strings of World Team Tennis, "Nasty" has had a poor year. Most people think his concentration is too flighty for him ever to win another major title, or even to come as close as he did here in 1972, when Stan Smith beat him in a magnificent final or 1976 when Borg demolished him in straight sets.

But when he is on, Nastase still has the feet of a zephyr, he touch of a gentle Latin lover. Yesterday was such a day as he coaxed soft, dipping service returns that forced Tanner to stretch for low volleys, and floated delicate lobs that danced wildly in the wind and fell in.

He also served 13 aces, won numerous other points outright onhis first serve and got away withe short, vulnerable second serves because Tanner was overhitting consistently. Nastase also returned well, keeping the ball low, at Tanner's feet.

Tanner got a bad call on the final point, argued to no avail with the umpire (It's frustrating when an official doesn't even acknowledge that you're talking to him"), and had the referee called on court. When he received no satisfaction, he shook hands quickly with Nastase, who had put on his warm-up jacker while waiting for a ruling, and hurried off court.

"(Nasty) tried to tell him the second serve was in. He was fine," said Tanner.

This match produced the forthnight's first example of a uniquely Wimbledon phenomenon: A crowd of people, perhaps 2,000 strong, gathered in the promenade outside the huge green walls of the Center Court, unable to see a shot but passionately following the match by a score flashing on the electronic scoreboard outside.

This was a pro-Nastase gathering, largely peopled by schoolgirls who squealed and cheered on each point he won and groaned on each one he lost. This produced an echo effect poosible only here, the delayed reaction of the spectators outside following by several seconds that of the live gallery at Centre Court.