Four excellent lead performances, vividly evoked ambience and a masterfully sustained mood of quiet desperation mark “Sydney” as an impressive piece of work. Despite a couple of plot twists that strain credibility, debut feature by writer-director Paul Thomas Anderson likely will generate favorable critical response. Pic has definite sleeper potential, but will need careful handling.

Drama begins with John (John C. Reilly), tapped out after an unlucky run at the Reno gambling tables, broke and shivering outside a roadside diner. A courtly stranger, Sydney (Philip Baker Hall), happens by, brings the poor wretch to his feet and takes him inside for a cup of coffee.

Sydney slowly gains the younger man’s confidence and offers to take him on as a protege. Despite his initial wariness, John is in no position to refuse the offer. They return to Reno, where Sydney, sounding at once paternal and cryptic, teaches John how to look and behave like a high roller. The lessons have an immediate payoff: John is offered a free hotel room by the casino management.

Two years pass, and John has proven to be an attentive student. The two men have developed a kind of father-son relationship, with the older man occasionally checking his protege’s excesses. Sydney disapproves of John’s friendship with Jimmy (Samuel L. Jackson), a vulgar and boisterous Reno regular. But he is sympathetically supportive of John’s infatuation with Clementine (Gwyneth Paltrow), a cocktail waitress with a serious self-esteem problem. When Sydney learns the woman moonlights as a hooker, he promises her that he won’t tell John.

Unfortunately, just a few hours after she and John are joined in a quickie marriage, Clementine picks up a customer in a casino bar, which leads to complications involving all four main characters.

Anderson makes the most of an obviously limited budget, and does a skillful job of conveying the tawdry, open-all-hours atmosphere of Reno’s casinos and restaurants. In this, he gets great help from the fluid cinematography of Robert Elswit, who manages a few quietly dazzling and dramatically effective tracking shots.

Despite the busy Reno backdrop, however, “Sydney” is basically a chamber drama, an anxiously claustrophobic four-hander that focuses almost exclusively on Hall, Reilly, Paltrow and Jackson. There is a hint of David Mamet in some of the edgy dialogue, especially in the confrontations between Sydney and Jimmy.

Anderson does his film a disservice two-thirds of the way through by introducing an “explanation” for Sydney’s paternal interest in John. Less damaging, but equally contrived, is a final scene that is too self-consciously ironic.

Hall, heretofore best known as the Nixon of Robert Altman’s “Secret Honor,” plays the title role with grave dignity and sad-eyed melancholy. His performance is nothing short of mesmerizing. Better still, Hall projects just enough authority and strength to make Sydney’s behavior in pic’s final third believable.

As John, a dim bulb who nonetheless is a basically decent fellow, Reilly is affecting and sympathetic. Paltrow deftly maneuvers through Clementine’s sudden mood swings, and is heartbreaking as she conveys the woman’s chronic self-destructiveness. Jackson steals every scene that isn’t nailed down with a performance that occasionally recalls the flamboyant menace of his “Pulp Fiction” portrayal.

Given its setting in a gambling mecca, “Sydney” doubtless will face many comparisons to “Leaving Las Vegas.” Taken on its own terms, though, this is an engrossing drama that, despite some narrative flaws, should satisfy venturesome audiences with a taste for indie fare.