Once Vikings players finally don pads on Sunday, the clock starts ticking down from six weeks until the season opener in Tennessee.

Only then, following two days in helmets and shorts, will the competition begin in earnest. And there may not be a more intriguing race this summer than the one set to spark on the offensive line.

Five starters — Matt Kalil, Alex Boone, John Sullivan, Brandon Fusco and Andre Smith — took their posts during the first walk-through on Friday. Less than two hours into training camp, Zimmer was asked if that's the starting lineup.

"Five guys we had will probably change, number one," Zimmer said. "And number two, I've talked to you about having competition this whole time and I'm not going to give you five guys after a walk-through. We still have a lot of time to figure it out. And until we get into pads, I'm not going to make any decisions on anything."

The preseason balancing act of deciding a starting five pits internal competition against seeking a set group to establish continuity. Snaps are up for grabs especially at right guard and right tackle, though the competition has been thinned early.

Phil Loadholt's retirement this week widened the lane at right tackle for Andre Smith, who signed a one-year, $3.5 million contract in free agency. T.J. Clemmings worked as the second-team right tackle on Friday.

Last year's starting right guard, Mike Harris, began camp on the non-football illness list with what he described as a head ailment. Harris figured to be Brandon Fusco's top competition at right guard.

Questions linger around center John Sullivan's health as he'll suit up this weekend for the first time since last August. Sullivan, 30, underwent two back surgeries last year to repair a herniated disc. He's currently backed up by 34-year-old Joe Berger, who started every game for Sullivan last season and played well.

Now establishing a starting five will be one of the coaching staff's most critical decisions this preseason.

"As soon as possible," Zimmer said. "It may be this week, it may be next week. I don't know. It's really about when we've made the decision that this is how we want it to go. We still have some combinations we want to look at. That is not the set lineup by any chance or stretch of the imagination. But yes, to answer your question, as soon as possible."