Clarke brought a six-shot lead into Saturday's third round, but tallied four bogies in his first seven holes. By No. 7, Clarke lost his entire lead, but the No. 17 player in the world is only one back heading into the final round.
These two separated themselves from the field, but a few players have a chance on Sunday. Rod Pampling shot a 3-under 68 and is alone in third place at 6-under-par 207.
Five-time Heritage winner Davis Love III carded a 2-under 69 and has sole possession of fourth place at minus-5.
They will be chasing Lonard on Sunday.
He broke into red figures with a birdie at the par-5 second, then chipped in for birdie at the fourth. Lonard collected his second birdie in a row at six when he missed the green at the par-5 hole, but chipped his third to tap-in range.
While Lonard was playing steady golf, Clarke could not find a fairway. He finally found the short grass for the first time at the eighth, but by that time Clarke tallied four bogeys and a birdie and was tied with Lonard for the lead.
Clarke seemed to get back in gear at nine when his sandwedge approach stopped 3 feet from the cup. He sank that putt, then tapped in a short birdie putt at 10 to go two clear of Lonard.
Clarke's putter let him down on No. 11 as he three-putted for bogey from 20 feet. Lonard drew even once again at 13 when he ran home a left-to-righter from 4 feet.
The par-5 15th hole proved to be a turning point. Neither played the hole well but had a pair of birdie putts. Lonard drained his 56-footer to reach 11-under par, but Clarke had 45 feet to match his playing partner. Unfortunately Clark hammered his putt 7 feet past the hole, then missed the comeback putt to fall two off the pace.
Clarke answered at 16 with a 12-foot birdie putt, but Lonard recaptured his two-shot advantage with a 10-foot birdie putt at 17.
Lonard made a mess of Harbour Town's closing hole. His 5-iron approach came up short and right of the green, then he chipped to 15 feet. Lonard two-putted for bogey, giving him the one-shot edge going into the final round.
'It all fell into place, so it was a good day,' said Lonard, who is winless on the PGA Tour and took the 54-hole lead for the first time in his career. 'Yesterday I did what Darren did today. I didn't drive it that great early on, and I got a little frustrated and held it together. I'm looking forward to tomorrow.'
Clarke still has a good shot at getting his third win on the PGA Tour. His previous victories came at World Golf Championships events, but the Ulsterman hopes Saturday was just one bad day.
'If you ask most guys in a tournament, they'll have three days where they play well and one day where they play a little bit not too well,' said Clarke. 'Hopefully this is my day when I played not so well.'
Jose Maria Olazabal (69), Stephen Ames (69) and Loren Roberts (68) share fifth place at 4-under-par 209.
Jim Furyk, the 2003 U.S. Open champion, shot an even-par 71 and is tied for eighth place with Joey Snyder III (69), Bart Bryant (70) and Jimmy Walker (71). They are knotted at 3-under-par 210.