Clarke began on the 10th tee at Harbour Town and parred his first five holes before a birdie at the par-5 15th moved him into the lead at 7 under par. Clarke then birdied the par-4 16th and added a birdie at the par-4 18th to make the turn with a three-stroke advantage.
'Sunday at Augusta, I worked on a couple of things and felt very comfortable there and hit the ball very well on the last day,' said Clarke. 'Actually I wasn't releasing the club enough, and I worked on it there and I worked on it here, so, so far, so good.'
The Ulsterman kept on rolling with a birdie at the par-4 first after his approach stopped within inches of the cup and made it three straight with a birdie at the par-5 second.
Clark missed the green with his second shot at the par-5 fifth, but he played his third within 2 feet and tapped in for another birdie. Clarke then missed the green at the par-3 seventh and knocked his second shot to 7 feet. Clarke drained the par putt and parred his two remaining holes to take a commanding lead into the weekend.
'Just keep going,' said Clarke, a two-time winner on the PGA Tour. 'Just keep doing what I'm doing and try to make as many birdies as I can.'
Lonard torched Harbour Town with 11 birdies on Thursday, but the Australian had to scramble in the second round. He also started on the back side and tallied a pair of birdies and a pair of bogeys over his first nine holes to remain at 9 under.
He stumbled to a bogey at the first, but recovered with back-to-back birdies from the second. Lonard then bogeyed four of his last six holes to finish six shots off the pace.
'It's not discouraging, I just didn't drive it good,' said Lonard. 'I feel like if I go out to the range and sort my driver out and drive it a good bit tomorrow, I'll be back in with the show.'
Sheehan started on the back nine and found trouble out of the gate with a bogey at the 10th. He recovered with a birdie at the 16th and reached minus-6 with a birdie at the first. Sheehan faltered with a bogey at the fifth, but responded with a birdie at the ninth for a round of 70.
'You're going to make mistakes out here,' said Sheehan. 'This course will test your patience, but if you keep playing well, I mean, anything can happen out here because any hole can grab you.'
Five-time champion Davis Love III picked up two birdies, an eagle and a bogey over his first nine holes. He struggled down the stretch with three bogeys and a birdie, however, for a round of 70.
Love was joined by Jim Furyk, Jimmy Walker, Rod Pampling, Woody Austin, Cameron Beckman and Nick O'Hern at 3-under-par 139.
Defending champion Stewart Cink finished in a group at 2-over-par 144 after a round of 73.
The 36-hole cut fell at 5-over-par 147 with 76 players qualifying for the weekend. Among those who failed to make the grade were Bernhard Langer, Lee Janzen, Mark Hensby and Ryan Palmer.