Injury-prone Bjorn Borg withdrew yesterday from the star-crossed World Championship Tennis (WCT) Finals because of an infected right thumb.

Borg, 21, had been scheduled to play Italian and Australian open champ Vitas Gerulaitis last night in a semifinal match in this $200,000 play-off for the top finishers in a January-through-April series of WCT-promoted tournaments.

Gerulaitis, 23, thus advanced by default to tomorrow's final, which pays $100,000 to the winner and $40,000 to the loser. Borg collected fourth-place money of $12,000.

Corrado Barrazzutti, the Italian "Little Soldier" who upset Brian Gottfried Thursday night, and Eddie Dibbs, who routed Ilie Nastase, were paired last night in a best-of-five-set match to determine Gerulaitis's opponent for the title match.

The usually prestigious tournament already had lost much of its luster when defending champion Jimmy Connors and Sandy Mayer, who qualided for the elite field, declined their invitations.

Connors apparently opted out because of a disagreement with WCT officials, reportedly over their refusal to [WORD ILLEGIBLE] him an appearance guarantee above the regular prize money. His mother and manager, Gloria Connors, [WORD ILLEGIBLE] that he had prior commitments. Mayer declined because he did not have a release from his contract with the Golden Gaters of World Championship Tennis.

Borg has defaulted frequently from tournaments the past three years, [WORD ILLEGIBLE] recently from WCT's $175,000 [WORD ILLEGIBLE] event last month because of the infected toe that was subsequently lanced in an operation in the [WORD ILLEGIBLE] Swede's adopted hometown of Monte Carlo.

[WORD ILLEGIBLE] said the thumb ailment is [WORD ILLEGIBLE] arisen from a blister.

"When I arrived here Sunday, I had [WORD ILLEGIBLE] but it didn't bother me much. [WORD ILLEGIBLE] plaster, tape, those kind of [WORD ILLEGIBLE] to take care of it," he said.

The thumb swelled badly after [WORD ILLEGIBLE] devastated Dick Stockton, 4-6, [WORD ILLEGIBLE] , in the tournament opener [WORD ILLEGIBLE] night. He had treatment [WORD ILLEGIBLE] tournament physician [WORD ILLEGIBLE] an injection of penicillin [WORD ILLEGIBLE] , but the pain and swelling has decrease, he reported.

[WORD ILLEGIBLE] me a little bit against [WORD ILLEGIBLE] Wednesday, it started hurting on Thursday, when I practiced with Barazzutti, I had to stop after 15 more minutes because I couldn't hold [WORD ILLEGIBLE] . Today it was worse . . . I [WORD ILLEGIBLE] even put my thumb on the [WORD ILLEGIBLE] without very sharp pain," Borg said.

[WORD ILLEGIBLE] yesterday afternoon, saying [WORD ILLEGIBLE] going to Monte Carlo, where he had have a complete physical [WORD ILLEGIBLE] to determine if there is a [WORD ILLEGIBLE] explanation for the [WORD ILLEGIBLE] . He said he would decide [WORD ILLEGIBLE] the week if he was fit for his scheduled tournament: the Italian in Rome, beginning May 22.

"I had lots of blisters the last [WORD ILLEGIBLE] five weeks. I don't know why. [WORD ILLEGIBLE] had this problem in my life," he said.

The skeptics have become leery of [WORD ILLEGIBLE] list of injuries, ailments and [WORD ILLEGIBLE] . They exist, no doubt, many wonder if they are really [WORD ILLEGIBLE] - given the realization [WORD ILLEGIBLE] , in tennis, a default can ruin a tournament, depriving spectators of the attraction they have paid to see.

Borg defaulted a round-robin match in the Colgate Grand Prix Masters in January because of the flu. He had already qualified for the semifinals, and subsequently beat Guillermo Vilas before losing to Connors.

Last September, he pulled out of the U.S. Open in the final set of a close match against Stockton because of a shoulder injury.

AT Wimbledon in 1976, he claimed to be close to defaulting with pulled stomach muscles before each of his last four matches, but served and smashed ferociously after having injections, and won the first of his back-to-back titles. The next week, he defaulted a Davis Cup match and was sidelined seven weeks.

Many times he has claimed injury after defeats - so frequently, in fact, that some have concluded he needs physical problems as an excuse to rationalize possible failure.

Thursday night, Barazzutti made a stirring revival to beat Gottfried for the third time in four career meetings - 6-4, 1-6, 6-1, 6-3 - and Dibbs reeled off the first five games and final nine in an stonishing 6-3, 6-0 devastation of the erring and listless Nastase.

Dibbs and Nastase had not played since the same time last year, when Dibbs won in four sets amid fireworks: profane words and gestures that threatened to flare into a fist-fight. There was hardly time for such incidents this time, Dibbs attacking every short ball in a relentless ground assault, capitalizing on Nastase's numerous unforced errors, and romping in only 84 minutes.

Barazzutti took hours to complete his resurrection against Gottfried, surging through nine straight games from 0-1 in the fourth set to 3-0 in the fifth, after losing nine of 10 to that stage.

Barazzutti escaped four break points from 0-40 in the fifth game, as Gottfried regrouped for a final stand, and shrugged off a couple of outrageously bad line calls to serve out the crazily twisting match.