Tiger Woods tightened his grip on the 89th USPGA with a controlled 69 to take a three-shot lead into the final round at Southern Hills Country club.
The 31-year-old Woods is closing in on a 13th major title
The world number one has won all 12 of his majors when leading going into the last day and is white-hot favourite to claim back-to-back USPGA titles.
Canada's Stephen Ames (69) will play with Woods after reaching four under.
American Woody Austin ended three under, Australian John Senden was two under and Ernie Els was one under.
Woods climbed into the lead with a major record-equalling round of 63 on Friday and he remained rock solid in front as his challengers wilted around him.
The 31-year-old edged to seven under with an eight-foot birdie putt on the 4th and touched eight under with a 25ft effort on the 12th for a five-shot lead over Ames.
Ames will have the dubious honour of playing with Woods on Sunday
Verplank, two shots adrift overnight, traded two bogeys with two birdies on the front nine playing with Woods but ran up a double bogey via the trees on the 12th and from then on it was hard work for the Oklahoma local.
The 43-year-old Austin made a late charge with back-to-back birdies on 15 and 16 but blew his chance to play in Sunday's final group with a bogey on the last.
Woods bogeyed the 14th before signing off with four straight pars but saw his long birdie putt on 18 lip the hole as it had done when he missed out on an historic 62 on Friday.
"The whole idea was to shoot under par and increase the lead and I did that," said Woods, who is chasing Jack Nicklaus's record of 18 major titles.
Woods has won all seven of the majors he has lead after 36 holes, while all six majors that have been held at Southern Hills have been won by the halfway leader.
"To have a great year you have to win a major. I've had some chances but haven't done it - I have a chance now and hopefully I can get it done," he said.
Ames, who suggested Woods was having trouble off the tee and was beatable in the WGC Match Play last year before losing 9&8 to the world number one, admitted he would have his work cut out to over turn the three-shot deficit.
"He's going to be a tough cookie to catch," said Ames. "Tiger has been in this situation a lot more than me but I'm happy to be there and I'm looking forward to it. You never know what can happen."
He [Woods] has got to make a lot of mistakes and I've got to play unbelievable
Three-time major champion Ernie Els showed more encouraging signs of a return to form following some lean years with a round of 69.
"I'm making a lot of soft mistakes but I did some good things, too," said Els, who was fourth in the Open at Carnoustie.
Els admitted the tournament was Woods's to lose but insisted he would still be battling on Sunday.
"The statistics will tell you it's over but as a competitor I can't say that. There are 18 holes left to be played," he said.
"He has got to make a lot of mistakes and I've got to play unbelievable."
American Boo Weekley rocketed into contention with an impressive 65 to set the clubhouse mark at level par, enough to earn him a tie for sixth at the end of the day.
The 34-year-old Floridian admitted he was unaware that a 25ft birdie putt on the 18th would have made him only the 24th player in major history to shoot 63.
"Really? Naw, I didn't know," said Weekley, who won his first PGA title in April at the Heritage.
"That would have been nice. I was just trying to make par and look how it ended up, trying to be safe."
Sweden's Peter Hanson (69) was the highest-placed European at two over, while Justin Rose (70) and Luke Donald (70) were the best Britons at three over.
World number three Phil Mickelson's woes continued as he slumped to a five-over 75 for seven over, while double US Open champion Retief Goosen carded 74 for five over.
Spain's Sergio Garcia was disqualified for signing an incorrect scorecard after playing partner Weekley put him down for a par instead of a bogey on 17.