Steve Stricker is the third round leader at the Memorial Tournament at Muirfield Village despite dropping three shots.
But back-to-back eagles enabled the American to end the day on 12 under, three clear of Jonathan Byrd, with Matt Kuchar and Brandt Jobe on eight under.
Northern Ireland's Rory McIlroy is a further shot back after a round of 71.
World number one Luke Donald had a mixed day, shooting a one-over 73, to lie eight shots off the lead.
Stricker is seeking his 10th US PGA tour crown, his first since his successful title defence at last July's John Deere Classic, and looked in great form as he began his round.
Starting with a three-shot cushion from round two, he eagled the 2nd and 5th holes before bogeying the 6th, but responded with birdies at the 7th and 8th.
But a lot of that good work was undone in the closing stages with bogeys at the 15th and 16th.
He was indanger of dropping another shot at the 18th when he drove into the middle of a sand-filled divot off the tee and then sent his approach shot into thick rough behind the green. However, he chipped out to three feet and managed to sign off with a nerve-calming par.
"All of a sudden, things seemed pretty hard, when at the start of the day, things were really going my way," he said afterwards.
"But I understand that, and I understand that's the nature of this game. So you add them up at the end, and it was a 69. And it was a good score."
Byrd had a much more steady round, working his way up the leaderboard into secod place from two under at the turn, with just one bogey at the 11th before ending the day with a birdie.
Kuchar had an inconsistent day, reeling off four birdies in a row at one point before bogeys at the 15th and 18th.
McIlroy had an extremely mixed day, including a double-bogey at the 4th and an eagle at the 15th. He birdied the 6th and 13th holes but carded a bogey at the 11th.
"Even though I didn't play my best stuff today, I'm still in a decent position going into tomorrow, and that's all you can ask for," he said afterwards.