Matteson was at 13-under 129.
Maggert's bogey-free round of 9-under 62 moved him from 39th place after the first round to a tie for second. He birdied half the holes at TPC Deere Run, including five of his last nine.
Harman had a second straight 65, the last two of his seven birdies coming after a 48-minute thunderstorm delay.
Matteson bogeyed the par-4 18th after getting four birdies in the first 17 holes.
He one-putted 10 greens and saved par twice en route to matching his career-low round. He finished by making a 13-foot birdie putt on the par-4 ninth hole.
"I saw the putts better today," Maggert said. "But I'm going to be aggressive tomorrow. I feel I need another really low round Saturday or Sunday, 7- or 8-under, to be in contention. The last few years, guys have really been lighting it up here."
Including Maggert's 62 and Matteson's first-round 61, there have been eight rounds of 62 or lower at Deere Run in the past five years, and 11 since the tournament moved to the course in 2000. The best effort was Paul Goydos' Tour-record-tying 59 two years ago.
A trio of players are at 10-under 132, including three-time defending champion Steve Stricker, whose 4-under 67 placed him within striking distance with 36 holes to play. Tommy Biershenk and two-time U.S. Open champion Lee Janzen are also at 132.
Zach Johnson shot 6-under 65 and was at 9-under 133.
Harman, in the next-to-last group, finished off his 65 by chipping in for birdie on the ninth hole.
Henry was 7 under in his first 11 holes, then finished with a 64 despite soft conditions that allowed players to aim shots directly at the hole.
"You jump the leader board when you least expect it," Henry said. "I was able to make that run today. You make par here, you get run over."
Christian, a 40-year-old Tour rookie, said that his 5-under-par 66 was achieved "the way I envisioned playing. This would be the nearest thing to approach being in the mythical `zone' -- but not quite there."
An eagle on the par-5 second hole jump-started his day. A bogey on the par-4 15th marred his scorecard, but he's closer to the lead than he's been all year, a season with only two top-20 finishes in 18 starts. His best finish was a tie for 14th at the Humana Classic in January.
Garrigus added a 66 to an opening 65.
Stricker's quest for a fourth straight John Deere title included five birdies after a bogey on his second hole of the day.
"I like the spot I'm in," Stricker said of being three strokes behind. "I'd rather kind of be there at this point."
After the round, Stricker went to the putting green to work on his stroke, despite taking only 29 putts.
"I'm hitting better putts," Stricker said. "I feel my confidence coming back. I'm standing over putts thinking I've got a good opportunity to make `em, even from 20 to 30 feet."
The cut came at 4-under 136 and encompassed 77 players, including amateur Jordan Spieth at 5-under 137. Former major champions John Daly, Stewart Cink, David Duval, Shaun Micheel and Todd Hamilton missed the cut. Bryce Molder withdrew after an opening 74.