Kirk Triplett fired a 5-under 67 and is tied at 17-under-par 199 with Brian Gay, who double bogeyed 18 for a round of 68. Jeff Sluman is one shot further back at 16-under-par 200.
Verplank entered the round three shots clear of Gay and Todd Barranger. Verplank got hot early as he rolled in three straight birdies from the fourth. He followed that with another birdie at the par-3 eighth to make the turn at 21-under.
On the back side, Verplank stretched his lead to four strokes with birdies at Nos. 11 and 12. However, his lead would shrink to two as he played the final six holes at the English Turn Golf and Country Club at 1-over. His bogey at the 16th was his first since the third hole during his opening round.
'I understand that the way the golf course is, you have to keep making birdies,' said Verplank. 'If I'm playing good, I'm going to hit a lot of fairways, a lot of greens and I'm going to have a lot of birdie putts. That is what I will try to continue to do tomorrow.'
Estes made a late charge to get into contention. He birdied the second and sixth to make the turn at 14-under.
'I had plenty of chances out there. If I was putting better, I could have easily shot 62,' said Estes. 'There's a lot of guys that can say the same thing, I suppose. The rough is not too penal and the fairways are wide and there's plenty of short holes.'
Around the turn, Estes drained a birdie putt at the par-5 11th. He leapt into contention with three straight birdies from the 14th before closing with back-to-back pars.
'My plan was to get to 19-under and try to get in the last group,' Estes said. 'I was trying to birdie 18 to get myself in the last group tomorrow, but my birdie came up just short. I sure would rather be at 19-under.'
Triplett made his move early in the round but was unable to sustain his momentum. He birdied the second before running off three straight from the fifth. Around the turn, he managed just one birdie at the par-5 11th before parring the final seven holes.
Gay had a chance to be in the final group on Sunday, but he faltered late. He had three birdies on the front nine and three more on the back side to get to minus-19. However, he double bogeyed the 18th hole to fall into a tie for third.
J.L. Lewis and Jerry Kelly each shot 7-under 65s on Saturday to get to 15-under-par 201. They share sixth place with Mark Wilson, who shot a 69 in the third round.
John Riegger (68), Cameron Yancey (68), Stuart Appleby (63), Chris DiMarco (65) and Steve Flesch (65) are tied for ninth at 14-under-par 202.
K.J. Choi, the 2002 winner, stumbled to a 1-over 73 to fall into a tie for 50th at 8-under-par 208.