After 13 days there are no pretenders left, only the two men who would be king of the world's clay courts. And so Bjorn Borg and Guillermo Vilas, the only true contenders for the No. 1 ranking in their sport last year, will meet this afternoon in the final of the French Open tennis championships.

They made a mockery of yesterday's semifinals, Borg conceding only a measly 36 points in routing Corrado Barazzutti of Italy, 6-0, 6-1, 6-0, and Vilas thrashing a bewitched, bothered and bewildered Dick Stockton, 6-3, 6-3, 6-2.

There has never been a more embarrassingly lopsoded semifinal day since the French, the paramount clay court test, went international in 1921.

Never has a man lost as decisively in the last two rounds as Barazzutti did yesterday. The previous most one-sided result was last year's final, in which Vilas annihilated Brian Gottfried, 6-1, 6-0, 6-0.

Vilas, the powerful but sensitive Argentinian lefthander lost only one set in seven matches in acquiring his first major title here a year ago. This time he sweated more, losing two sets to Billy Martin in the second round, one to French junior Yannick Noah in the third, and two to Chilean Hans Gildemeister in the quarterfinals.

By contrast, Borg, the glacially cool Champion of 1974-75 who did not play here last year, has hardly perspired. Winner of the Italian Open two weeks ago, while Vilas was off winning a smaller tournament in Munich, he went through the top half of the 128-man draw like Gen. Sherman marching through the South leaving devastation in his wake.

The 22-year-old Swede won six matches in 18 straight sets, losing only 27 games. Only two sets were closer than 6-3, both of them in a 6-2, 6-4, 7-6, victory over Roscoe Tanner in the fourth round.

Borg, who won Wimbledon last year for the second straight time and was ranked by most knowledgeable experts as No. 1, even though Vilas took the French and U.S. Opens and compiled the longest winning streak of the open tennis era (50 matches), must be favored in the final.

He has beaten Vilas in 12 of their 16 career meetings, dating back to 1973. Borg has won the last five, eight of the last nine and all the important ones since 1974. They met here in the final in 1975, Borg winning, 6-3, 6-2, 6-4.

Both players are on waves of confidence. Vilas captured the German Open at Hamburg before going to Munich. Borg was impressive in Rome.

They are the toughest back-court players in the game, tireless thumpers of heavy topspin off both wings, but Borg has more flexibility in his game. He has a better serve and volley, plus the knowledge that he is in peak form and the psychological edge of having "owned" Vilas for so long.

It is difficult to imagine anyone playing better on a slow court than Borg did yesterday in ravaging Barazzutti in just 80 minutes.

The Italian "Little Soldier" has not come close to beating Borg in eight meetings. He did not surrender before the match, as some expected, but he might as well have. Borg made him look like a boot camp private.

Even in the prematch warmup, the icy Swede had a lean and hungry look. The court was watered and rolled hard - it almost looked like a rust-colored cement court for a couple of games, until roughed and scuffed with skid marks - and Borg came out attacking, going to the net more than he would against a less defensive opponent.

He held at love in the first game and broke Barazzutti in the second, a game that turned out to be prophetic. Barazzutti started with a double fault, then Borg slugged a forehand approach winner and a backhand volley after another good approach, and got the break on an unforced backhand error. After that, the deluge.

Barazzutti had a break point in the third game - one of only three against Borg in the match - but he knocked a forehand return long. Then Borg creamed another forehand winner and an overhead to escape.

Vilas was not nearly as overwhelming, but Stockton collapsed completely after getting his only break of the match, for a 3-2 lead in the second set. Vilas double faulted twice in that game, committed a backhand error, then netted a backhand passing shot off a short, rather poor approach. He looked vulnerable.

But Stockton immediately fell behind, 0-40, failing to follow three first serves to the net, and after getting back to deuce, netted a backhand volley and passing shot, both bad errors.

Vilas held at 15, and Stockton lost his seve again from 40-15, netting an easy forehand, sailing a backhand long and netting a high, simple forehand volley on the last three points. He was never a factor after that.

The 27-year-old New Yorker-turned-Texan was hesitant about getting to the net, the only position from which he really could pressure Vilas. He made so many unforced errors it became painful to watch.

By the end, Stockton looked groggy.The match had become an ordeal for him and for the 12,504 spectators at Stade Roland Garros, who took to whistling and hooting because the tennis was so bad.

If these semifinals had been a Broadway show it would have closed. But the final today could be a hit.