Justin Thomas carded a 65 on day three to get to 15 under, but countryman Patrick Reed and South Africa’s Erik van Rooyen were just one back after an enthralling third day in Mexico City.
Jon Rahm carded a record-breaking 61 to sit four shots off the lead alongside World No 1 Rory McIlroy and 36-hole leader Bryson DeChambeau.
Rahm’s round – his lowest as a professional, a new course record and the joint-lowest round in WGC history – contained a stunning hole-in-one on the 17th, the second of the day after Chez Reavie had earlier aced the third.
His 11 under total looked like it would still leave the Spaniard with a mountain to climb as Thomas made five birdies on the back nine, but he and Reed both bogeyed the last to leave the door ajar.
Reed and Van Rooyen both carded rounds of 67, while McIlroy signed for a 68 and DeChambeau – who had led by three at one point – bogeyed three of his last six holes in a 71.
Thomas already has a World Golf Championships title to his name and has every right to feel confident going into round four, having finished in the top 10 all three times this event has been held at Chapultepec.
‘I’ll just go out and play golf and keep doing what we’re doing,’ he said. ‘For the most part I feel like I’m playing well, doing all the right things.
‘It’s just my bogeys this week have been sloppy, so I’d really like to have a clean card tomorrow and not have any of those and just give ourselves a good chance the last couple of holes.’
Van Rooyen won his first European Tour title last season to finish tenth in the Race to Dubai Rankings Presented by Rolex, but this is his first time in the final group of a WGC, although he insists the occasion will not get to him.
‘You’ve just got to go play golf, man,’ he said. ‘You can’t overthink this kind of stuff. At the end of the day, there’s 18 holes in front of you and you’ve got to navigate it the best you can.
‘I can’t control what Justin Thomas does, he’s a phenomenal player. I can’t control what Patrick Reed does or the guys behind us. Jon Rahm just went out and shot 61 today. Who’s to say one of us is going to win? All I know is I’m going to focus on myself and do my thing.’
He made the most of the 11th before putting an approach to three feet at the 12th and, while a poor chip had him bogey the next, he made a fourth chip-in of the week on the 14th and capitalised on the 15th.