As Rory McIlroy carded a one under 70 to fall back to joint third, Justin Thomas fired a five under par 66 to take a one-stroke lead over world number one Dustin Johnson and set up a tilt at his fourth win of the PGA Tour season in the WGC-Mexico Championship.

The 23-year old, who has converted his last three 54-hole leads into victories, leads on 12-under par and now has a chance to become the 12th winner under the age of 30 in 16 PGA Tour events this season if he can overcome final round partners McIlroy and Johnson.

“It's cool playing in the final group with DJ and Rory," Thomas said. "That's pretty cool. That's something I haven't experienced before, playing with those guys.

"I've obviously played with Hideki and Rosie, I've played with a lot of great players, but it will be a new experience.  I've just got to go out and we've got to stick to our game plan.

"I was really proud of my round today because I started hitting it pretty bad again there in the middle and just kind of found it again or just was trying to manage it and brought it home strong.”

The highlight of Thomas' round was a hole-in-one at the 232-yard, par-3 13th hole where his six-iron hopped high in the air short of the flag and went straight into the hole on the fly. 

As for McIlroy, he was frustrated by his round alongside Mickelson (68) and Ross Fisher (72), which featured four birdies, three bogeys and seven missed greens.

"That was a kind of weird day," McIlroy said. "It was boring and frustrating in many ways. I felt like I couldn't get into any sort of rhythm and it was just sort of, it was all a bit slow, and I was trying to get something going but just couldn't quite do it.

"I felt like there was a good chance around the turn to get something going. I made birdie on 11, I drove it way down there on 12 and then hit the pin with my second shot and came back. One of those days, just couldn't really put like a few good holes together I guess."

Johnson, who fired a second consecutive five-under 66 to head into the final round one stroke behind Thomas, won in his last start with a five-stroke victory at the Genesis Open.

The US Open champion is looking to join Tiger Woods as the only player with four or more World Golf Championships victories while a win would also see him claim his 14th PGA Tour title at age 32 years, eight months and 11 days.

Johnson is also seeking to join Thomas (3) and Hideki Matsuyama (2) as multiple winners on the PGA Tour this season and become the fifth player to win his first PGA Tour start as world No. 1 following Ian Woosnam (1991 Masters Tournament), David Duval (1999 BellSouth Classic), Vijay Singh (2004 RBC Canadian Open) and Adam Scott (2014 DEAN & DELUCA Invitational).

“I hit the ball great on Thursday, I just didn't make any putts," said Johnson, who lost a ball in a tree at the 16th but got up and down from 154 yards for bogey, holing a 14 footer. 

"Yesterday I played really solid, today I played really solid. Obviously everyone's struggling a little bit on the greens but I feel like I'm hitting the ball really good.

"I'm rolling it good, too. I made a few putts to start the round early I think.  I think I made more feet of putts on the first three holes than I did in all three rounds combined.”

Johnson (13/2008-present) has at least one win in each of his first ten seasons on the PGA TOUR (the best current streak) and joins Arnold Palmer (17/1955-1971), Jack Nicklaus (17/1962-1978) and Woods (14/1996-2009) as the only players since 1960 to achieve this feat.

Last season, he claimed the Jack Nicklaus Trophy as PGA Tour Player of the Year and also took home the Byron Nelson Award for Adjusted Scoring Average (69.172) and the Arnold Palmer Award as the Tour’s leading money-winner with $9,365,185.

As for McIlroy, he has exceeded expectations in his first start since suffering a rib injury seven weeks ago.

Despite being forced to settle for a 70, he's still just two strokes behind Thomas.

“Hopefully that’s the bad round if the week out of the way but I felt terrible out there today compared to the first couple days, especially yesterday when I felt like everything was clicking," McIlroy said.  

"Didn't feel so good today.  But it's nice to still be in the last group for the final round - I can keep an eye on what the guys ahead of me are doing and try and catch them up.”

The 2016 FedExCup champion and a four-time major champion, McIlroy is seeking his third World Golf Championships title having already won the 2014 Bridgestone Invitational and the 2015 Dell Technologies Match Play. 

According to the PGA Tour, the 27-year old from Co Down would return to No. 1 in the Official World Golf Ranking with a win this week, as long as Johnson finishes worse than solo fourth. 

He would also overtake No. 2 Jason Day, absent from this event due to illness, with a two-way tie for third or better.

McIlroy finished tied third in the event at Doral last year after holding a three-stroke lead over Adam Scott and Dustin Johnson heading into the final round. He was also third in 2012.

If he wins tomorrow, he will pip Johnson to the 14 PGA Tour wins mark. 

There's also a chance that 46-year-old Phil Mickelson, a two-time World Golf Championships winner (2009 Mexico Championship, HSBC Champions), can take his 43rd PGA Tour after rounds of 67, 68 and 68  left him in a tie for third with McIlroy, just two shots off the lead

Mickelson birdied the 15th, 16th and 17th to reach 11-under before a final hole bogey dropped him two strokes back.

The left-hander holed out from off the green three times in the third round for birdies at the first and fourth holes and for par at the 14th, bringing his total number of hole-outs this season to 12 (three at CareerBuilder Challenge, two at AT&T Pebble Beach Pro-Am, and four at Genesis Open).

Mickelson has proven his mettle on high altitude golf courses with his win at the 1990 U.S. Amateur at Cherry Hills as well as the 1993 and 1997 International at Castle Pines in Denver, which are located approximately 5,300 and 6,300 feet above sea level, respectively. 

This week, Club de Golf Chapultepec is located at 7,800 feet above sea level.

Mickelson is searching for his first win on the PGA TOUR since the 2013 Open Championship.

Lee Westwood (66) and Jon Rahm (67) are just three off the lead in joint fifth on nine under.

One of six players making their World Golf Championships debut this week, 22-year-old Rahm was No. 137 in the Official World Ranking before winning the Farmers Insurance Open to advance to No. 46. 

Two more strong performances (T16/Waste Management Phoenix Open, T5/AT&T Pebble Beach Pro-Am) moved him to No. 37 helping secure his position inside the top 50 and earn a spot in the field at the Mexico Championship.

Rahm is making his 20th PGA TOUR start and owns six top-10 finishes, including a win (2017 Farmers Insurance Open), a runner-up finish (2016 RBC Canadian Open) and a third place result (2016 Quicken Loans National).

Rahm said: "It was a little bit of up and down. I started a little, just a little weird. I was hitting good shots, feeling good, but the ball wasn't doing what I wanted and I'm three over for three holes. Then I kind of pulled it back. Hit a great drive on four, made a birdie and that kind of got the round going.

"I feel like it's really hard to make birdies on the front nine. Luckily I was able to hit a couple of them close on the back nine and made some birdies."

Europeans Tommy Fleetwood (67), Tyrrell Hatton (68) and Thomas Pieters (68) are tied seventh on eight under with Jordan Spieth up to tied for 10th on seven under following a course record, bogey-free 63.

Spieth's round included six birdies and an eagle two at the 322-yard par-four first hole (his 10th), where his tee shot finished over the green and he chipped-in from 23 feet.

With his recent win at the AT&T Pebble Beach Pro-Am, Spieth tied Jason Day for most victories since the start of the 2013 PGA TOUR season. 

Spieth and Day have nine each, while McIlroy is joined by Johnson, who now owns seven after his Genesis Open win.

World Golf Championships-Mexico Championship, Club de Golf Chapultepec, Mexico City (Par 71, 7,330 yards)

1 Justin Thomas 69-66-66—201 (-12)

2 Dustin Johnson 70-66-66—202 (-11)

T3 Rory McIlroy 68-65-70—203 (-10)

T3 Phil Mickelson 67-68-68—203 (-10)