Dustin Johnson on his way to an impressive round of 67 at the WGC - Mexico.

• Tiger Woods stalking leaders in WGC Mexico Championship 
• Ricky Fowler penalised after falling foul of new rules

How thoughts of a procession can quickly evaporate. Rory McIlroy, four holes into his second round at the WGC-Mexico Championship, had already backed up an opening 63 with another three birdies. His lead over the field was four.

Even a dropped shot at the 6th, after McIlroy found water when trying to reach the par five in two, did not particularly register. Worse, as he afford hope to the remainder of the field, was to follow. McIlroy four-putted from the fringe of the 9th for a double bogey. He missed from 3ft at the 15th. All the while, Dustin Johnson and Matt Kuchar lurked with intent.

McIlroy remains a key part of this tournament, his second round of 70 meaning he is still very much in contention but success will not come as easily as had appeared. Johnson has opened up a two-shot lead.

There would of course be eyebrows raised should Kuchar prevail again in Mexico; an almighty rumpus developed after he low-balled a local caddie on victory in this country three months ago.

Tiger Woods backed up a level par 71 with a 66, which leaves the 14-times major champion with hopes of a weekend leaderboard assault. “The winning score here really hasn’t been all that low,” Woods said. “You can get it going pretty good around this course, which is fun, but also it can go the other way without it taking much.

“I got a little more familiar with the course. I got a better feel for the tee shots, the lines, approach shots and the greens. It helped to have that one look in a competitive environment. Overall this was a pretty good day, I got a lot out of the round.”

Woods offered rare personal insight when assessing how his level of patience, as is required at this venue, has risen since childhood. “It wasn’t my natural strong suit, that’s for sure,” he added. “My dad was a very patient man and I learned by being around him. As I matured by being around this game, I got a little more patient; especially since having kids, that definitely will test one’s patience.”

Sergio García dropped a shot at the last but can still be content with a 66 that moved him to minus seven.

Rickie Fowler fell victim to the long arm of the golfing law after being penalised for an illegal drop on his first hole of the day. Fowler shanked his second shot out of bounds with his drop taken from shoulder rather than knee height, as per new rules introduced in January. Fowler signed for a 73 and 36-hole total of plus two.