The United States' Thomas - ranked world number four - is one of golf's hottest players and has already won the Tournament of Champions this year.

His round featured eight birdies and two bogeys and he reached the Club de Golf Chapultepec clubhouse on -15.

Fellow American Reed and South Africa's Van Rooyen each shot 67 and will begin the final day on -14.

Reed lost to Thomas in a playoff at the Tournament of Champions and while the pair are both major winners and household names in the golfing world, Van Rooyen has only one European Tour victory to his name.

World number one Rory McIlroy, who is seeking to complete a WGC grand slam in Mexico, is four shots off the lead on -11. 

American Bryson DeChambeau, the leader at the halfway stage, joins him after a level-par 71, whilst Jon Rahm is also on that score after a blistering round of 61 - a new course record.

The Spaniard shot nine birdies and a hole-in-one eagle in a sensational round.

Englishmen Tyrrell Hatton and Paul Casey are both on -10.

Leader Thomas has won one WGC tournament before, the Invitational in 2018.

He also reached a playoff at the WCG-Mexico Championship the same year, losing to compatriot Phil Mickelson on the first hole.

Asked about his plan for the final round, Thomas said: "Just go out and play golf and keep doing what we're doing.

"Try to get off to a little better start on one, but for the most part I feel like I'm playing well, doing all the right things."

Thomas, Reed and Van Rooyen will be the final group, with Rahm, DeChambeau and Northern Ireland's McIlroy making up a penultimate three-ball sure to also draw a big crowd.