Victor Pecci, who had been the most surprising and swashbuckling overachiever in tennis the past month, today joined the unprecedented list of seeded casualties in the wild Wimbledon tennis championships - blackjacked by a gambling, go-for-broke young Australian named Brad Drewett.

On a day when the blue-ribbon entries for the men's singles title - defending champions Bjorn Borg, John McEnroe and Jimmy Connors - finally hit championship stride, Pecci, Brian Gottfried and John Alexander joined the astonishing first-week scrap heap that now contains 10 of the 16 men's seeds.

Borg - whose celebrated pulled thigh muscle turned out to be a phantom, and rumors of his imminent withdrawal from the tournament a flaming false alarm - feasted on the big serve of Californian Hank Pfister with devastating returns and literally sprinted to a 6-4, 6-1, 6-3 victory.

No. 2 seed McEnroe and No. 3 Connors, who likewise had sputtered through their first two matches, also moved into higher gear and won impressively - McEnroe over left-hander Tom Gullikson, 6-4, 6-4, and Connors over speedy South African Johan Kriek, 6-4, 6-1, 7-6.

But all around them, the upheaval of a memorable first week at the All England Lawn Tennis and Croquet Club continued. Never before have 10 seeded players lost in the first week of Wimbledon - and we have not yet gotten to the first Saturday, traditionally the tournament's most notorious afternoon for upsets.

Pecci, 23, had never reached the third round at Wimbledon before, but was justifiably seeded No. 8 on the basis of his recent form, which had been as glittering as the diamond stud he wears in his right ear.

He made the largest quantum jump in the history of the computerized world rankings - from No. 30 to No. 15 - after bearing four seeds, including Guillermo Vilas and Connors, to reach the final of the French Open three weeks ago on Parisian clay. Then he was runner-up in a grasscourt tournament at London's Queen's Club, taking a set from McEnroe in the final as he had from Borg in Paris.

Today, "Diamond Vic," as the British have come to call him, started well enough on the fast turf of Court No. 1. But he lost two marathon tie breakers - 11 points to 9 in the second set, 10-8 in the third - and fell rather unluckily to the 20-year-old Drewett, 4-6, 7-6, 6-4.

Gottfried - a quarter-finalist here last here who was seeded No. 9 this time - continued the fitful form he has shown since taking six weeks off when his first child was born on Easter. He came out on the short end of a duel between two straightforward serve-and-volleyers, losing to improving Brian Teacher, 6-3, 7-5, 5-7, 6-3.

Alexander, a sturdy Australian bred on grass courts and seeded No. 11 served poorly and never got into his match against Sandy Mayer, who victimized him with steadier service returns and ground strokes, 6-4, 6-4, 6-4.

Since No. 4 seed Vitas Gerulaitis, No. 6 Guillermo Vilas, No. 7 Arthur Ashe, No. 10 Wojtek Fibak, No. 12 Jose Higueras, No. 13 Manuel Orantes, and No. 16 Corrado Barazzutti had already been uprooted prematurely, large sections of the draw are now wide open for unexpected flowers to blossom.

Going into Saturday's fourth round, these are the men's singles pairings in the top half of the draw: Borg versus Teacher, Sandy Mayer versus Tom Okker (a semifinalist last year who today ousted Tim Wikison, 7-5, 7-5, 6-4), Connors versus Mark Cox (who saved five match points and beat Frenchman Gilles Moretton today, 3-6, 6-7, 6-1, 7-6, 7-5), and Drewett versus Bill Scanlon.

In the bottom half McEnroe plays No. 15 seed Tim Gullikson, the righthanded twin of the man he beat today; No. 5 Roscoe Tanner, who has not lost a set, faces No. 14 Jose Clerc (a 6-7, 7-6, 6-3, 7-5 victor over 1972 champ Stan Smith); Bob Lutz opposes Pat DuPre, and Adriano Panatta takes on Gene Mayer, Sandy's younger brother.

By contrast, the women's singles to the equivalent stage has been rather boringly formful, an indication that women's tennis at the moment still lags far behind the men's in depth, if not star quality at the zenith.

The only seeded women who have lost are No. 12 Sue Barker and No. 13 Regina Marsikova, beaten today by fellow Czech Hana Mandlikova, 6-38 6-2. Only two of the 16 third-round women's matches played today went to three sets - Betty Stove beating Kay McDaniel, 2-6, 6-3, 6-2, and Debbie Jevans beating Amanda Tobin 3-6, 7-5, 6-2.

Going into Round Four, the women's pairings: defending champion Martina Navratilova versus Greer Stevens, Stove versus Dianne Fromholtz, Tracy Austin versus Virginia Ruzici, Mandlikova versus Billie Jean King, 1977 champ Virginia Wade versus Jevans, Kathy Jordan versus Evonne Goolagong, Wendy Turnbull versus Kerry Reid, and Laura Dupont versus No. 2 seed Chris Evert.

As a chill wind and overcast preempted the sunshine that had prevailed the previous two days, there were lots of curious matches on another of Wimbledon's uniquely crowded and bustling afternoons.

For example, Panatta - the dashing Italian - beat 6-foot-5-inch Swede Ove Bengtson, 7-6, 7-6, 7-6, on Court No. 13, which was so chopped up there hardly seemed to be any bounces there at all.

Panatta seved 19 double faults, but only one a tie breaker. He won 25 of 28 points on his serve in the three tie breakers, which he won, 15-13, 9-7 and 7-4. CAPTION: Picture, Defending champion Bjorn Borg makes a return to Hank Pfister whom he defeated, 6-4, 6-1, 6-3.UPI