Wendy Turnbull admitted that when the U.S. Open began last week, she didn't in her wildest dreams imagine that she would be spending this Saturday afternoon in the stadium of the West Side Tennis Club, playing Chris Evert in the final.

"I very rarely look ahead and see who my next opponent is," the ascending 24-year-old Australian said today after thrilling a crowd of 12,128 by outplaying No. 2 seed Martina Navratilova, 2-6, 7-5, 6-4, to become the most surprising finalist here since Carole Graebner in 1964.

"When I came to Forest Hills, I knew I had to play Sabina Simmonds in the first round. I asked my doubles partner, Cynthia Doerner, who she played in the second round. She said me. I didn't even know."

Turnbull, a 5-foot-3, 118-pounder who is so quick around the court that colleagues call her "Rabbit," knows whom she plays next for the $33,000 first prize and a title that no one thought was within her grasp: Evert, who beat Wimbledon runner-up Betty Stove today, 6-3, 7-5.

Evert - the champion here the last two years and winner of 112 consecutive matches on clay courts dating back to August, 1973 - was far from peak form in the one-hour 21-minute match.

She had difficulty gauging the breeze that came and went, sometimes blowing briskly, on a day of radiant sunshine. She was never able to groove her usually unshakeable groundstrokes against the 6-1, 165 pound Stove's hit-or-miss, rhythmless game.

And although she refused to dwell on it, Everet apparently was bothered by an ailing left arm that hurt her on her two-fisted backhand. She touched it gingerly several times during the match but said at a postmatch press conference, when asked if there were anything wrong with her arm, "No everything's fine."

Trainer Bill Norris later confirmed, however, that Evert "is experiencing a little nerve-pinching in her upper left arm." She received three treatments today, and will have one more and a massage before the final.

"I don't want to talk about it. I don't want my opponents to know about it.I'll talk about it after the tournament," Evert said privately.

"I don't think it will effect her much, but she is concerned about it," added Norris. "She doesn't say anything because she doesn't like to have any excuses. She's a gamer."

Evert was never behind in either set but she kept getting ahead and then losing her serve to let Stove catch up - usually in long, ragged games.

From 0-3 in the first set, Stove got back to 3-3, then lost her serve at 15 - sailing a backhand long after three errors into the net - and let four break points get away as Evert held for the set after five deuces.

There were seven service breaks in the second set. Evert led, 2-0, 4-2 and 5-4, and each time Stove leveled, then promptly played a terrible game to lose her serve.

In the fifth game, she double faulted at 15-40; in the ninth, she netted a forehand volley and made three forehand groundstroke errors to lose her serve at love; in the 11th, after saving three match points on Evert's serveM she double-faulted to 15-40 and netted an easy forehand.

Stove had a break point 30-40 and then saved two more match points as Evert served out the match the second time she tried.

This was Evert's toughest match here since the 1975 final when she topped Evonne Goolagong, Evert lost only 12 games in six matches last year and had just lost 10 in beating Sharon Walsh, Pam Whytcross. Helena Anliot, Nancy Richey and Billie Jean King to get to the semis.

"I don't feel that great about my game," said Evert, who had trouble with her service test, did not move to the ball particularly well and was thrown off by Stove's well-disguised drop shots and changes of speed and spin. "I think I'll have to play a lot better in the final."

When she said that, she expected her opponent would be Navratilova, who served and volleyed superbly in the first set, then became overly cautious.

Turnbull, 24, was seeded No. 12 but only started the best top players during rthe recent World Team Tennis season, after she was traded by the Boston Lobsterz in the Cleveland Nets for Navratilova and became the No. 1 singles player.

She had never before played on center court here, in the 54-year-old concrete horseshoe with 10 granite eagles perched as sentries, and was at first overwhelmed.

"I found it very hard to concentrate the first set," Turnbull said, "because I was thinking, 'I'm out in the stadium, playing the semifinals in Forest Hills?' Martina was all over me, attacking my backhand, and I really couldn't do anything. I felt like a bit of a fool out there."

The gusty wind and noise from planes taking off from nearby LaGuardia Airport were also unsettling, but when Turnbull got plugged into the match, she played well and intelligently.

Turnbull has a compact, controlled game. She likes to slice her ground strokes deep, setting up her opponents for a blistering forehand or a sneaky-fast backhand that she slides down the line. She rallies patiently but will swoop to the net if the path looks safe.

She uses the entire court, drop shots and approaches well, and her swiftness unnerved opponents. She also served smartly today, working on Navratilova's backhand and then poping one hard to the forehand when it was least expected. She got three key aces that way.

"I started serving a lttle better toward the end of the second set, tossing the ball higher and hitting out, and then rest of my strokes kind of fell in place," she said.

Turnbull had three break points in the second game of the second set - two in the fourth, one in the eighth, and finally broke for the set at love in the 12th.

That game - in which she ripped two return winners and forced and error after Navratilova had netted an easy forehand - was the start of a run of nine straight points. Turnbull led, 3-0 and 5-1, playing positively to get to the verge of victory, and then lost her serve twice.

Navartilova saved a match point at 40-30 on Turnbull's serve in the ninth game, cranking a backhand, down-the-line winner that clipped the sideline.

Two points later, she had the break back to 4-5, but Turnbull shoved her to 0-40 in the next game with a backhand, down-the-line pass and cashed her third match point, jumping on a forehand return of a second serve and coming in behind it to knock off a forehand volley down the line.

"I got a little tight, she started to hit out a bit more, and when she saved the first match point I thought I had lost it," said Turnbull. "I still can't believe I'm in the final."

Evert, who had a 3-0 lifetime record over Turnbull and beat her four straight times in team tennis play this year, is hungry for this title after losing to Virginia Wade in the semifinals at Wimbledon. "I think it would make my year," shesaid.