The magic of their performances, both singly and together, had completely redeemed Tuesday's laskluster opening night, until the physical hazards of the profession so suddenly intervened.

Fate decided to play killjoy last night at the Kennedy Center-where American Ballet Theatre was presenting the season's second performance of Mikhail Baryshnikov's production of "Don Quixote" - by dropping a muscle spasm on lead dancer Antony Dowell just at the very climax of the evening, thereby casting a pall over what until that moment had been a superlative performance.

When the music introducing the male solo variation in the Grand Pasde Deux of the ballet's last act began, dancer John Meehan, in the supporting role of the matador, Espada, stepped on stage instead of Dowell and commenced the steps, to the mystification of the audience. Meehan finished out the variation and the ensuing, extremely difficult coda, and Dowell never returned to stage.

Afterwards it was learned (though never announced to the crowd) that Dowell had suffered a severe ankle cramp during the entree, or first part, of the pas de deux, and was unable to continue dancing.

A hint of trouble had appeared in the adagio section when Dowell had some uncharacteristic trouble supporting his partner Gelsey Kirkland in several of her turns. But the substitution by Meechan, no doubt made in a split-second decision backstage, took place so smoothly that probably, many people hardly realized anything had happened. Richard Schafer, in the role of one of Basil's companions, slipped into Meehan's matador part during the last few minutes of the act.

Dowell was treated backstage with heated compresses, and company spokesmen did not seem to feel that the condition very serious. It was, however, too soon to tell last night what the consequences might be for Dowell's further schedule performances during the four-week run.

The incident was an eerie echo of the sad mishap last March, only a few days after the world premiere of the Baryshnikov production at Kennedy Center, when ballerina Martine van Hamel fell on stage during Act I, sustaining a serious injury which kept her from performing for months, and required a series of instantaneous cast changes involving Baryshnikov, Krkland, Cynthia Gregory and others to keep that and subsequent performances going. John Meehan was dancing the romantic lead, Basil, opposite van Hamel when the accident occurred.

Dowell, the masterful classical dancer who took a year's leave form England's Royal Ballet to join ABT this past September, was giving his debut performance as Basil last night, partnering Kirkland. The magic of their performances, both singly and together, had completely redeemed Tuesday's lackluster opening night, until the physical hazards of the profession so suddenly intervened.

Dowell, noted for his noble reserve and elegance, proved a remarkable comic talent in the part of the rapscallion barber who woos an innkeeper's daughter, and his dancing was flamboyantly virtuosic. Kirkland, who was the original Kitri, burned up the stage with her seductive tempests. And the pairing of the two was pure alchemy, promising much for the partnership's future. But everything about the performance-until the ill-fated moment at the end-clicked in all the respects that were missing from opening night. The music too, under John Lanchberry's direction, resumed its remembered sparkle.

Dowell's original schedule called for a "Giselle" Friday evening; a "Bayadere" Sunday evening; and another "Don Quixote" Dec. 16. Reports of his condition are expected today. CAPTION: Picture, Antony Dowell