Rory McIlroy cut Dustin Johnson's advantage to two shots as the final round of the WGC-Mexico Championship got under way at Chapultepec Golf Club.
The American was on course for a sixth World Golf Championships title as he entered day four with a four shot lead but a bogey in his first three holes dropped him to 15 under.
Playing partner McIlroy picked up a birdie on the second to move to 13 under, three shots clear of American Patrick Cantlay and four ahead of Englishman Paul Casey.
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.
Northern Irishman 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.
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 another on the sixth.
Sergio Garcia, Cameron Smith and Tiger Woods were then seven shots off the lead.
