A band could study for four years with the most distinguished professors at the most prestigious music school in the world and probably not achieve the kind of chemistry that country stars Tim McGraw and Faith Hill showed Friday night at Xcel Energy Center.

After they sang a brand-new duet, “Break Fast,” they stood at center stage looking eye to eye. McGraw smiled at his wife, she made some kind of gesture, and he broke out laughing.

“I lost the stare-down so quickly,” he announced, “that nobody knew we had a stare-down.” Then he added, “Any time your wife stares at you for any length of time, you know your [goose] is cooked.”

Suddenly it turned into the “Sonny & Cher Show.”

Hill explained that “my husband’s butt is smaller than mine.”

Retorted McGraw: “This is going off the rails quickly.”

It did seem that way at first. And not because of the repartee.

The sound mix was so out of balance for the first half-hour that neither McGraw’s nor Hill’s voices came through loudly and clearly. Their two backup singers sounded more forceful. And the band, which sometimes featured five guitarists, was just too overpowering.

Moreover, Hill, 49 — who hasn’t released an album of new material since 2001 or toured since their last duo trek in 2007 (somebody had to help their three daughters with their schoolwork) — sounded road-weary vocally. Yet, the crowd of 15,000 responded enthusiastically to her hit power ballad “Breathe” and the way she blasted “Piece of My Heart,” the Janis Joplin rock chestnut.

The fans cheered even louder for McGraw, 50, who has released five studio albums and performed six times in the Twin Cities since the last McGraw-Hill tour. He also seemed to get more artful staging, complete with lasers and fancier lighting.

His 2004 anthem “Live Like You Were Dying” was the predictable crowd favorite, but he also impressed on a solo a cappella ad-lib of Prince’s “Purple Rain” and an acoustic, harmony-heavy reading of “Where the Green Grass Grows.”

Frankly, the buff country superstar seemed way less dynamic than he has in his own shows in the area. He seemed to be more into muscle poses than expending energy.

At least Hill took the initiative to stroll through the arena’s main floor as she sang her 2005 signature “Mississippi Girl.” Then her hubby one-upped her on the very next song by showing up in the back of the arena and mixing it up with the fans as he sang “Something Like That” all the way to the stage.

Unlike the two previous McGraw-Hill tours, this one did not have an in-the-round stage, but one at the end of the arena. This show was less effective than their last appearance, even though it was similar in format: Duets, Hill solo, McGraw solo and more duets.

Friday’s two-hour performance, which will be repeated Saturday at the X, ended with an alternative version of the stare-down. Hill and McGraw sat face to face on two chairs, a hand on the other’s knee, sharing a single microphone in a stand.

The song was their 2007 Valentine “I Need You,” as romantic and steamy a duet as they’ve had in their repertoire (though they promise a duets disc by the end of this year). They looked into one another’s eyes and crooned with their most nuanced and impassioned vocals of the night.

At song’s end, Hill rolled her fists playfully like she was ready for a fight. She burst into a big smile, put her mitts down and then kissed McGraw for the first time all night.

That’s the kind of chemistry you get after being married for nearly 21 years.