Harman had two eagles in a 6-under 65 on Saturday that gave him a one-stroke lead after three rounds of the John Deere Classic.
Harman's solid performance got him to 17-under 196 at TPC Deere Run. His partner on Sunday will be three-time Deere winner Steve Stricker, who is all alone in second, one stroke better than Scott Brown.
''I pay attention to what Steve's doing but don't base my game on his,'' said Harman, who was grouped with Stricker and Johnson Wagner in the third round. ''He's had a lot of success here.''
Stricker, who shot a 7-under 64 for a three-round total of 197, is 116 under par in his last 23 rounds at TPC Deere Run, including victories in 2009, 2010 and 2011. Brown is third after a 10-under 61, matching the best round on the PGA Tour in 2014.
Two years ago, Harman was paired with eventual winner Johnson, though not in the final twosome.
''I watched Zach win this golf tournament, and learned a lot,'' Harman said.
Stricker, making his ninth start of the year, isn't making room for another Deere trophy just yet.
''So many guys are in this thing,'' Stricker said. ''Guys can get off to hot starts here, but you can answer them also.''
There are 10 players within four shots of the lead and 13 within five. But of those players, only Stricker and Johnson, tied for fourth at 14 under, have won on the course.
''It will still be difficult for me tomorrow,'' Stricker said. ''I'm not in that final group a lot out here, especially now. I'll want to take advantage because I'm not playing very much. To get back into the winner's circle would mean a lot.''
Stricker's last win was the 2012 Tournament of Champions. His best finish this season is a tie for sixth at the Memorial. But Harman has never finished better than a tie for third.
The 27-year-old Harman hit 17 greens and 12 of 14 fairways in the third round. The left-hander from Georgia bogeyed Nos. 12 and 18, the latter by missing the green with his approach. His eagles came from 30 feet on the par-5 second and from 47 feet on the par-5 17th.
Stricker had his streak of bogey-free holes end at 24 with a 5 at the par-4 11th, but rallied for birdies on the last two holes.
On a day when players were allowed to clean their ball in the fairway because of an early morning downpour, Brown flirted with a 59, but had to settle for birdie on the par-5 17th after reaching the green in two shots, and then parred the par-4 18th.
Harman's long eagle putt on the second hole triggered his nice round.
He had birdies on the fourth and eighth holes to go out in 31, and then added two more birdies before his long downhill eagle putt on the 17th, which turned left in the last 15 feet before falling into the cup. His bogey at the final hole, coupled with Stricker's birdie there, trimmed his three-stroke lead to one shot.
Brown's 61 came after what he called a ''bad 1-under'' 70 on Friday.
''I was just trying to get back into the golf tournament, and it turned into a great round,'' Brown said.
Johnson's 2-under 69 extended his streak of under-70 rounds at par-71 Deere Run to 23.
Defending champion Jordan Spieth shot 67 and is tied for 14th at 202, six strokes behind Harman. Spieth came from six off the pace last year to win.