The way Martina Navratilova had been playing all week, people knew she had a chance to dethrone Chris Evert, the top-seeded defending champion, in the final of the $100,000 Virginia Slims of Washington tennis tournament.

But the 8,120 witnesses who showed up at Capital Centre at noon on Super Sunday could not have anticipated how the 20-year-old lefthander would totally devastate the No. 1 woman player in the world, 6-2, 6-3.

Navratilova, who at her best could be the most exciting player in women's tennis, has the type of explosive, attacking game that troubles Evert when Navratilova has all her potent offensive artillery firing together.

She had not beaten Evert since the first tournament of last year's Virginia Slims Circuit, at Houston 51 weeks agao, but yesterday she was fit, determined, and had all her power harnessed.

Their styles, and the overall esthetics of their games, couldn't be more different.

Evert, the statistician's heroine, is an institution at 22. She rolls alon like Old Person River, awesome in her consistency, collecting championships and prize money ($319,565 last year, more than $1 million in four seasons) as surely as silt gathers in a river bed.

Navratilova is normally erratic, and hasn't as yet cut any great gorges into the landscape of women's tennis, but on her day she is more spectacular and thrilling, all shooting rapids and white water.

Yesterday was one of those days, and Evert's baseline game was not enough of a raft to carry her through the jagged rocks. Navratilova punctured it with piercing winners - from the backcourt, the forecourt, and on the move in from one to the other. As the water swirled around her, even the usually unflappable Evert became impatient, even a trifle panic-stricken.

Evert was eventually drowned in a torrent of forehand ground strokes, sure and sometimes acrobatic volleys, and a ferocious aggression that Navratilova sustained from nearly beginning to end of the 57-minute match.

"This should quiet down the critics who said I am playing so bad because I'm lazy," declared Navratilova, who has lost 20 pounds (to a muscular 145) and dedicated herself to getting into good physical and psychological condition after a disappointing season. Last year was an evenful one for Navratilova as she defected from Czechoslovakia, was plagued by nagging injuries and emotional problems, and won only two tournaments, none between January and October.

"Because I kept so much pressure on her, she made some unforced errors she doesn't usually make," Navratilova continued. "She tried to make her shots too good. I was serving so well, keeping the pressure on, that it was easier for me to break her serve."

The volatile Navratilova, who has worked hard and successfully at curbing her temperamental outbursts on court, lit up in a huge smile as Evert smacked the last of numerous unforced forehand errors into the net to lose her serve after saving the first two match points against her.

Ever had played perhaps her best game of the afternoon to break Navratilova for the only time as the expatriate Czech served for the match at 5-2. But she immediately lost her serve for the third time in a row - the fiftn time in nine service games - to end it in a game that went to deuce twice.

Navratilova took a little while to settle down in the doubles final that followed, then volleyed superfbly and made the most of partner Betty Stove's forcing backhand returns from the deuce court to defeat Valerie Ziegenfuss and Kristien Shaw, 7-5, 6-2.

Navratilova was a perfect lefthanded complement to towering (6 foot, 160 pounds) Stove's righthanded power. They blended nicely and took advantage of southpaw Shaw's faltering backhand volley in the second set to split the $5,200 doubles top prize.

The $20,000 singles top rpize was presented in stacks of fresh one-dollar bills. "That's for 20 years of tipping, I guess," grinned Navratilova.

Understandably delighted at having beaten Evert for only the fourth time in 19 career meetings, the champion was bubbly and charming at the presentations, conducted by Katharine Graham, publisher of The Washington Post.

"I'm going to take a real quick shower so that we can get the doubles on and all go watch the Super Bowl," she told the crowd that had supported her. Obviously Navratilova has become as Americanized as Emily since defecting from Czechoslovakia to Beverly Hills and Palm Springs late in 1975.

Navratilova is the first repeal winner in the six years of the local Slims event. She won here in 1975 after beating Evert for the first time in the quarterfinals, a thriller that went to the ninth and last point of sudden death in the final set.

This time Evert - who, unlike Navratilova, customarily does not play her best at the start of a new season - never seemed as if she could make it that close.

Navratilova is hitting her forehand devastatingly again after a year of being inhibited by a painful wrist. The cross-court forehand is her best ground stroke, and it counteracts Evert's best weapon, her two-fisted backhand cross-court, which yesterday satisfied frequently.

Navratilova was so quick and agile at the net that she also took away Evert's passing shot off both wings. Discouraged, Evert played a little safer than usual instead of hitting out, and that only made Navratilova's net game more dangerous.

Evert got a higher percentage of first serves in - 73 per cent (29 of 40), compared to 66 per cent (35 of 54) for Navratilova - but her deliveries did not have nearly as much bite.

As Evert, who sensed she didn't have her usual control over her ground strokes, became increasingly hesitant, Navratilova expanded in confidence, both in the backcourt and at net.

Her backhand was merely adequate - she blocked or sliced it most of the time, hitting over the ball only occasionally - but her forehand was fearsome. She even blasted winners off Evert's sidespin forehand down-the line, which is difficult because the ball falls away trickily.

But Evert, who last year won 12 of 17 tournaments she played, 75 of 80 matches, and lost only one after April did not have it yesterday.

"Martina played super tennis, and I didn't play a good match," said Evert. "I think I could have been a little more gutsy, a little more patient, but I didn't hang in there. I got a little frustrated and wasn't willing to stay out there all day if necessary to win a point. I hit some shots hard and deep, and she hit winners off them. Martina was playing like she did in 1975."

Added Evert, "This is a changed Martina. She's lost weight, worked hard, and if she keeps playing like this she's going to be tough. She didn't play well at all last year, but I'm not going to be able to take her lightly this year. It's good for me because it gets my game up."