Johnson, the defending champion, shot a 5-under 66 to join Reed and Glover atop the leaderboard at 12-under 130 following second-round play.
It ''was a day of just hanging in there and letting things come. But I like the fact that I don't have to be perfect and I can still play here,'' Johnson said.
Reed shot a 63 in the morning session, just one year after missing the cut at TPC Deere Run – and Glover finished a shot better at 9 under on Friday.
Given how well Johnson has played this course of late, it could take a spectacular weekend to beat him.
Johnson has shot 18 consecutive rounds in the 60s at Deere Run – largely by avoiding big mistakes and scrambling out of small ones – and his ninth bogey-free round Friday was a tournament record.
''I'm just comfortable. I'm comfortable with every tee shot. I'm comfortable with every wind, and clearly I'm comfortable on the greens,'' Johnson said.
Consistency has been an issue this season for Glover, but he was just two feet from taking the lead outright heading into Saturday. He missed a 19-foot birdie putt on his final hole to join Johnson and Reed atop the leaderboard.
Glover said he ditched plans to take this weekend off and head to Scotland early for the British Open after playing poorly last week. So far, it looks as though a few extra rounds have helped Glover find his game.
''I was ecstatic (Thursday) after 3 under ... so you can imagine how I feel,'' Glover said. ''But I'm not dumb enough to think it's over.''
Reed, a 22-year-old with two top-10 finishes in 20 events this season, highlighted his strong round with a 37-yard chip for eagle on the 17th hole.
Reed, with his wife Justine on the bag as his caddie, is 13-under over his last 27 holes.
''She helps me with everything. I never check wind. She tells me what the wind is. She's always right on that. Most of the time she helps me pick most of my clubs. She seems to know my distances better than I do,'' Reed said. ''She's great at reading putts, so I kind of have the full package. It's definitely not an 'I,' it's definitely a 'we' for us.''
Matteson had his best performance of 2012 at Deere Run a year ago, when he pushed Johnson to a playoff and finished second.
Matteson's round on Friday was punctuated by an ace from 132 yards out, as he used a wedge to notch the tournament's first hole-in-one in three years.
''I just said, 'You know what? Let's just try to hit a good shot, kind of get some spin on it, bring it back down the hill and leave ourselves a good putt,'' Matteson said. ''It started coming back down the hill and it just disappeared, and the guys behind the green went crazy.''
Darron Stiles also aced No. 7, marking the first time in nine years that a pair of hole-in-ones was recorded on the same day at Deere Run.
Chez Reavie shot Friday's low round, coming in at 61 to tie the best second-round score in tournament history.
There could be more stellar performances like that in the final two rounds, given the shape of the course and forecasts of more ideal conditions on Saturday and Sunday.
''You'll see low scores from the beginning of the field through the middle and all the way to the end. You know, it's there for the taking,'' Glover said.