Dustin Johnson took a five shot lead into the final nine holes of the WGC-Mexico Championship as he stayed on course for a sixth World Golf Championships title at Chapultepec Golf Club.
Rory McIlroy had cut his four shot overnight lead to two in the early stages but the American exuded his usual calm and turned in 35 to stay at 16 under.
England's Paul Casey was six under through 12 holes to sit alongside McIlroy, one shot clear of American Patrick Cantlay.
Johnson is the only man to win all four WGC crowns and is looking for his second European Tour win in as many starts after his triumph at the Saudi International powered by SBIA.
McIlroy played a lovely chip from just short of the second green to set up a birdie and looked set for a two shot swing when Johnson found a greenside bunker with his second and left himself 18 feet for par.
The 34-year-old is known for his nerves of steel, however, and he rolled home to stay at 16 under.
He could not repeat the feat at the par three next, finding sand off the tee and doing well to get down in two from 55 feet.
McIlroy went down the right off the fourth tee and dropped a shot after failing to find the green with his second to put Johnson three ahead again.
The leader's tee-shot was behind a tree on the fifth but he got a free drop as he was stood on the cart path - McIlroy was not so lucky on the next.
The Northern Irishman had to play out sideways left-handed and found the water with his third but played a nice up-and-down to drop just a single shot.
It was an impressive bogey but Johnson punished it, holing from nine feet to open up a five shot lead.
Casey had matched the lowest round of the day on Saturday with a 65 and he was flying again, birdieing his first three holes and adding a further gain on the sixth to turn in 31.
He made another birdie on the tenth and when he took advantage of the par five 11th, he was in a share of second alongside McIlroy.
Cantlay found a greenside bunker with his tee-shot on the driveable first but got up and down beautifully for an opening birdie to move into double figures and he stayed there at the turn.
Justin Thomas was on course to match his 62 from round three of last season's event at nine under for both the week and his round with one to play.
Spaniard Sergio Garcia was also seven shots off the lead having played his first nine holes in level par.
Dutchman Joost Luiten was the clubhouse leader after a 64 as birdies on the fifth, sixth, ninth, tenth, 11th, 12th, 14th, 15th and 16th were book-ended by bogeys on the fourth and last.
