American Ballet Theatre put on its best and worst faces at the season’s first performance of “Giselle” on Monday night. The first act looked like it had been slapped on the stage, and much of it was conceived and performed along distinctly down-market lines. Act II, danced with care and conviction, seemed the work of an entirely different ensemble: This performance would not suffer comparison with any of the world’s major companies.

The cast was led by Julio Bocca as the aristocratic seducer Albrecht and Xiomara Reyes as the guileless peasant girl he ruins. Their work was generally excellent in both acts; the problem lay in the secondary roles and the orchestral playing.

Jennifer Alexander has often shown that she is an excellent dance actress, but she portrayed Albrecht’s noble fiancee, Bathilde, as a mincing, tail-switching minx who went into a petulant snit when she realized that a peasant dared to have designs on her fiance. She isn’t the only ABT Bathilde to behave this way; for the most part, the company has drastically misconceived this role in recent years.

As Bathilde’s father, the Prince of Courland, Victor Barbee suggested a swaggering Henry VIII. Mr. Barbee’s portrayal was rather startlingly ripe, but it wasn’t overripe. Carlos Lopez mimed convincingly as Albrecht’s squire Wilfred, and Gennadi Saveliev had the right rude glower as Hilarion, Giselle’s village suitor. As Giselle’s mother, Berthe, Susan Jones didn’t seem integrated to the other characters onstage.

Erica and Herman Cornejo’s peasant pas de deux should have been spectacular but was instead good with spectacular moments. Mr. Cornejo’s jumps and entrechats were as bouncy as ever, but his leg dropped instantly to the ground every time he landed from a leap – as was not true with Mr. Bocca, who is 15 years older – and he pounded the floor pneumatically.

Throughout the first act, conductor David LaMarche dashed through the score at double time, so that Ms. Reyes looked like an over-cranked wind-up doll in her solo. But she managed to do all the steps and finish with the music – an extraordinary feat under these circumstances. In the second act, Mr. LaMarche throttled back, and the score proceeded as it should. Only in Ms. Reyes’s Act II solo did Mr. LaMarche again accelerate markedly, and this was undoubtedly at her request. Ms. Reyes seemed to be intending it as an homage to her Cuban forebear, Alicia Alonso; Ms. Reyes was trained in the state ballet school in Havana that Ms. Alonso has directed for decades.

It may be that Mr. LaMarche was trying to do what Ms. Reyes wanted and simply got carried away in the first act. But stage rehearsals with orchestra were invented to avoid things like that, and ABT’s apparent willingness to forgo them is a case of penny-wise, poundfoolish.

Ms. Reyes had seemed like an unlikely choice to open the season’s run of “Giselle” performances, but she evidently prepared with tremendous diligence and gave the best performance I’ve seen from her. In the first act she was a bit coy, but her attempt to mold all her movement to the contours of Romantic style and her relatively restrained projection gave added credibility to her interpretation.

I was further impressed by Ms. Reyes’s ability to do justice to the unearthly dynamic extremes of the second act, when Giselle rises from her grave to both join the sisterhood of man-hating Wilis and to save her lover from their clutches. In the first “Vision” duet with Mr. Bocca, her extensions were a bit abrupt, but in the pas de deux her developpe was beautifully secure and unhurried. Even though she is shorter than the corps de ballet that surrounded her, Ms. Reyes commanded the stage as she should in Act II, moving with lyricism and sweep.

The role of Albrecht has never come easily to Mr. Bocca. In the first act he never convincingly portrayed an aristocrat disguised as a peasant: He was too emphatic, too rustic and realistic. But no duality exists in the Act II Albrecht, and Mr. Bocca was free to devote all his creative intentions toward the simulation of nobility, which he did convincingly. His emotion flowed freely and persuasively.

Mr. Bocca danced well in both his Act I and II solos – much better, in fact, than when I saw him last year. Just as important, he partnered magnificently: At all moments in the spectral second act, Ms. Reyes seemed elusive, weightless, and uncontained by time, space, or gravity.

Dancing Myrtha, the queen of the Wilis, Gillian Murphy contributed significantly to the high quality of this second act. The amplitude, legible intent, and inhumanly slow tempo of her gestures made her a riveting figure of authority. Malevolent she certainly was, but she was not a vampire. She stayed within the parameters of Romantic ballet at its most lofty and lyric.

Ms. Murphy’s performance evinced her newfound powers and confidence as a dance actress.When the efficacy of her myrtle scepter was defeated by Albrecht and Giselle’s unanimity – or their taking refuge by the cross over Giselle’s grave, depending on how you read the narrative – Ms. Murphy crumpled. For a moment, we saw the betrayed maiden she, too, had once been. But a moment later she was again icily indomitable.

As her lieutenants, Stella Abrera and Veronika Part each danced superbly, impalpable as rustles. All told, the corps of Wilis danced with a delicacy I hadn’t seen in ABT’s “Giselle” for a long time.

ABT’s Metropolitan Opera season runs until July 15 (Lincoln Center, 212-362-6000).