Had yesterday been a normal Sunday, Anne Smith and Martina Navratilova likely would have spent the afternoon together, glued to a TV screen, screaming wildly for the Dallas Cowboys to clobber the San Francisco 49ers.

On the women's pro tennis tour, Smith and Navratilova perhaps are the two most fanatical all-sport fans, both specializing in addiction to the Cowboys.

Smith, after all, is Dallas born and raised; as for Navratilova, the central project of her life--a very natural one, all things considered--is becoming an American spiritually as well as legally. When Navratilova defected from Czechoslovakia seven years ago at the age of 18, the Cowboys were America's team. So, touchingly, they became Martina's team, too.

Yesterday, however, was no normal Sunday. At least not for Smith and Navratilova.

"I don't think we'll watch it together," said Smith. "It wouldn't feel quite right."

The reason is that, tonight at 7 on the court at Capital Centre, Smith and Navratilova will meet in the final of the Avon Championships of Washington with a $40,000 first prize at stake.

Then, as an added twist, a half-hour after their singles match, which will be the biggest event by far in Smith's tennis life, the same two women will face each other again in the doubles final. Smith and her partner Kathy Jordan, the No. 1 seeds, will meet Navratilova and Pam Shriver, the No. 2 seeds, in a little $15,000 argument.

It will be a storybook match, thanks to Smith's presence. Already this week Smith's New Year's resolution--to make the world notice her as a singles player, not just a doubles wizard--has achieved ridiculous success.

Within four days, she upset the No. 2-, No. 7- and No. 6-ranked women players in the world, Tracy Austin, Pam Shriver and then Sylvia Hanika. If Smith were also to upend Navratilova, the No. 3-ranked woman, the tennis world would be sent scrambling to its history books to find out when, if ever, a player so unheralded had upset four of the top seven players in the world in six days.

With all this at stake, too much fraternization is out.

Especially for Smith, who never before has reached the championship singles match of any pro tournament and who must overcome the psychological weight of thinking about her past record against Navratilova, 0-12. Actually, Smith, ranked 16th in the world, has one victory over the great left-hander, but it came when Navratilova retired with an injury while leading the match.

On the surface, this Smith versus Navratilova business seems a mismatch. Last season, for instance, Navratilova won $865,437, the third consecutive year she had set a record for the most money ever won in a sport by any woman. By contrast, much of Smith's $192,311 in 1981 winnings (eighth best on the tour) came from doubles. Navratilova won so many titles, 10 last year, that even she can't keep track of them all. Before this week, Smith had only reached one semifinal in her pro life.

In addition, Navratilova always has treated the winter indoor Avon circuit as her own private bank account. Once, she reached the finals of 12 consecutive Avon events. If she beats Smith, it will be Navratilova's 10th Avon title in less than two calendar years.

As if this doesn't seem hurdle enough for Smith, Navratilova, except for a nagging sore throat for the past fortnight, is in splendid spirits and splendid form. "I'm just having fun," she says. "At this stage of my career, I've got all the shots. I've pretty much got it all . . . I can try to give the crowd pleasure and myself, too. I felt so much pressure at the end of last year, trying for the No. 1 ranking, that now I'm just enjoying the sport for itself."

That has made Navratilova a pleasure to watch during her Washington stay. As her semifinal victim, Barbara Potter, said, "Martina is our Hercules . . . She has that clay-courter's fluidity in all her strokes and her movements, but she's definitely a fast-court player because of her power . . . Her concentration is also underrated. For ability to concentrate under pressure, she's probably right behind Chris (Evert Lloyd) and Tracy (Austin)."

"I should send Barbara a bottle of champagne for saying that," said Navratilova with a laugh. "I really don't get much credit for having much mental discipline. People think that because I'm emotional on the court, because I cry (as she did in anger over a mistaken 'conduct warning' on Saturday), then I must be easily distracted."

As though this weren't enough on Navratilova's side, she also has gotten her first choice among opponents in the final. Before Smith beat Hanika on Saturday, Navratilova said, honestly, "Anne is a good friend of mine, and a Cowboys fan, too, so I guess I'd rather play her in the championship.

"I know what to expect when I play Anne. It will be a lot of serve and volley and not much changing of pace or spins. Against Hanika, it would be a lot of spins and slices. It should be easier to get in a groove against Anne because of her style."

And Navratilova in a groove in a battle of pace is a crushing opponent.

Despite all of these factors, however, tonight's final has a special piquancy that comes rarely in any sport. For one night, all the past history of one of the players, Smith, may be completely irrelevant.

Occasionally, for no apparent reason, an athlete arrives at a magic place. Tasks that, all their lives, seemed beyond them suddenly become simple. Usually, they don't ask questions. They just dance while the music lasts.

That's where Pepper Smith dwells now, in that fantasy zone where the previously impossible inexplicably seems almost easy.

When Smith takes the court tonight, even she won't know until the first balls are struck which Anne Smith has shown up.

Will she be the impatient, flawed half-player who achieves stardom only in doubles? Or will she be the serve-and-volley thrashing machine who dazzled Austin, Shriver and Hanika this week?

At least this time, Smith will be allowed to live her fantasy out to its conclusion. Once before, she reached a juncture when she had a chance to do something utterly incredible. And that chance was taken from her.

"In a high school basketball game, I once scored 50 points in the first half," said the slim 5-foot-5 Smith.

How many did she end up with?

"Fifty-four," said Smith.

What happened?

"The coach took me out," said Smith, placidly.

How did she feel about that?

Smith's face broke into a mischievous grin, the kind that Austin, Shriver and Hanika have recently learned to fear.

"Gee," said Pepper Smith, "I really wanted to go for 100."