Mexico City - Dustin Johnson edged closer to a 20th PGA Tour title on Saturday, firing a five-under par 66 to take a four-shot lead over Rory McIlroy heading into the final round of the WGC-Mexico Championship.

Johnson built a 16-under par total of 197 at Club de Golf Chapultepec, where McIlroy fired a three-under 68 for 201.

It was another three strokes back to their nearest rivals - Masters champion Patrick Reed, fellow American Patrick Cantlay, Spain's Sergio Garcia and Australian Cameron Smith - on day that saw Tiger Woods's challenge fade with a disastrous double bogey-bogey at the 15th and 16th.

Reed posted the round of the day with a seven-under 64. Cantlay carded a 65, Smith a 68 and Garcia a 69.

Johnson kickstarted his round with an eagle at the par-four second, where he hit his tee shot to four feet.

But his lead, up to as many as five, dwindled to two when he was in the trees on the way to a double-bogey at the 10th.

It was his first miscue of the week, and Johnson responded with four more birdies coming in to maintain a comfortable advantage.

"I was playing good," the American said.

"I played really well the front nine even though I missed quite a few short birdie putts.

"Even on 10, actually I had a decent shot at the flag. I just mis-judged the lie a little bit. It shot straight up in the air and hit the tree in front of me.

"I didn't let it bother me," Johnson said, "because I knew there was plenty of holes I could make birdie on coming down the stretch."

McIlroy had three birdies and three bogeys in his first nine holes - rolling in a 34-foot putt at the ninth to launch a run of three straight birdies.

After a short par putt lipped out at 14 he birdied 15 and 16 as he tried to keep Johnson in his sights.

"I battled," McIlroy said. "I probably didn't hit it as well as the first couple of days but I hung in there.

"Especially after the eighth hole, playing the last 10 holes in four-under par to keep myself within some sort of touch of DJ is nice."

McIlroy said the course, which already offers an unusual challenge of altitude situated at 7 800 feet above sea level, was getting more difficult as it firmed up.

"It's getting really bouncy," he said.

"The fairways are playing really more narrow. Some reactions on the greens are throwing people a little bit.

"It's going to be tricky tomorrow."

Woods, a seven-time winner of the tournament but playing it for the first time since it moved to Mexico, was a case study in how things can go wrong on the Chapultepec greens.

The 14-time major champion made an early run to move into the top five, but a four-putt double-bogey at 15 followed by a three-putt bogey at 16 saw him post a one-under 70 that left him 10 adrift on 207.

Johnson, seeking a fifth World Golf Championships title, said he won't change his approach on Sunday.

"I'm going to come out and play exactly how I've been playing the last three days," he said.

"Got a lot of great players right behind me, so I'm going to have to go out and shoot a good score."

Leading third-round scores on Saturday in the World Golf Championships Mexico Championship (USA unless noted, par-71):

197 - Dustin Johnson 64-67-66

201 - Rory McIlroy (NIR) 63-70-68

204 - Patrick Reed 72-68-64, Patrick Cantlay 72-67-65, Sergio Garcia (ESP) 69-66-69, Cameron Smith (AUS) 69-67-68

205 - Ian Poulter (ENG) 68-68-69, Kiradech Aphibarnrat (THA) 68-69-68

207 - David Lipsky 68-71-68, Louis Oosthuizen (RSA) 72-66-69, Tiger Woods 71-66-70, Tommy Fleetwood 70-65-72

208 - Paul Casey (ENG) 71-72-65, Xander Schauffele 69-73-66, Francesco Molinari (ITA) 71-67-70

209 - Lee Westwood (ENG) 73-71-65, Hideki Matsuyama (JPN) 72-70-67, Gary Woodland 71-71-67, Tyrrell Hatton (ENG) 67-70-72

210 - Danny Willett (ENG) 71-69-70, Kevin Kisner 73-67-70, Charles Howell 69-68-73

211 - Matthew Fitzpatrick 72-71-68, Tony Finau 73-69-69, Keegan Bradley 69-73-69, Branden Grace (RSA) 71-71-69, Rafa Cabrera Bello (ESP) 76-69-66, Bubba Watson 70-75-66, Richard Sterne (RSA) 69-68-74