American Idol: Casey James First on the Shania Train

Country night on American Idol has resulted in many a sad song over the years.

Not because the contestants were crooning Patsy Cline or Johnny Cash, mind you, but because a lot of otherwise grand ole finalists tend to tank when Nashville comes calling.

But if anyone could help more pop-minded artists polish their boots, it was tonight's guest mentor, pop-country superstar Shania Twain, who also helped pluck unknowns out of the ether during auditions in Chicago.

The chart-topping Canadian has been lying pretty low, musicwise, since going through a divorce in 2008, but she was more than up for the challenge of readying Idol's top six to perform her music on Tuesday's show.

Did they come through when the going got tough?

Lee Dewyze: "You're Still the One" sounded a little rough coming out of Lee's raspy pipes—but maybe the late-'90s love song was due for a little roughening up. He altered the arrangement just enough to please the judges, and it came out sounding like signature Dewyze rather than an anniversary-celebration staple. "The sound of your voice on every song makes it sound so relevant," Kara DioGuardi praised him before getting into a tiff with Simon Cowell over the "weird faces" Lee was pulling onstage. "He was smiling," Kara explained to the Brit, ultimately leaving him with no time for a real critique—though he did manage to call Lee's song choice "perfect."

Michael Lynche: Big Mike likes to pull at the heartstrings, doesn't he? He did the wounded teddy bear thing with "It Only Hurts When I'm Breathing," turning the country ballad into a believable, albeit pitchy, R&B tune. But maybe we were hearing things. Ellen DeGeneres compared his sound to Luther Vandross, and Simon called that analogy "spot on," though he called the performance "a bit wet." Which apparently means "girly" to you Yanks out there.

Casey James: Tasked with spicing things up this week after being accused of going through the motions, Casey hit it out of the park. He really wrapped his singing voice (which, in case you had forgotten, is quite nice) around "Don't," and left the image of a jamming-for-kicks bartender in the dust. Ellen, Randy Jackson and Simon called it one of his best performances to date, and Kara told him he'll "be in the front of this competition" if he keeps pushing the emotional envelope like that. Simon prompted him to head out in the audience and give Shania a proper thank-you for all the help—and he did.

Crystal Bowersox: Unsurprisingly, Crystal slipped seamlessly from her usual Joplinesque rock-godessness into Tammy Wynette territory with "No One Needs to Know," making the tune even more country than usual. Which, surprisingly, left the judges strangely cold. It was unanimous that this wasn't Crystal's best performance (duh), but Simon put it more bluntly: "Shocker, we don't like Crystal this week." The feisty single mom fought back, though, pointing out that just because the previous songs were "bigger" didn't mean they were "better."

Aaron Kelly: The kid actually made "You Got a Way" sound less country—although, according to Randy, Aaron is the show's country artist this year. We'll admit, we totally forgot that about Aaron amid all the easy-listening love songs. "Tonight, you were like a different artist," Simon said, calling the performance "sincere" and "believable." "Tomorrow's going to be real tough," Ellen said of the looming elimination. And Kara, who never met a visceral performance she didn't like, was wowed as well. She even applauded the now-17-year-old for changing the line "the way we make love" to "the way you show me love." (Well sure, they don't want to get raided.)

Siobhan Magnus: "Any Man of Mine," one of Shania's most upbeat, cheer-worthy songs, oddly made for one of Siobhan's flattest performances to date. Sure, she moved around and looked happy, but the vocal was all over the place. Her scream lost a little steam at the end, too. But apparently we're hallucinating. Three of four judges loved it ("way to pull the Shania Twain into the station," Ellen quipped), and Simon thought it was "fun"—though he did point out that Siobhan sounded "almost as if [she] was giving birth" at the end.

One of these six is going home tomorrow. Who do you think has reached the end of the line?

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Check out some of Shania's country cohorts looking lovely in our 2010 Academy of Country Music Awards gallery.