Golf-Cold putter prevents McIlroy from challenging in Mexico

March 5 (Reuters) - Rory McIlroy never looked like he would regain his world number one ranking on Sunday as a cold putter prevented him from mounting a challenge in the final round at the WGC-Mexico Championship. Two strokes from the lead starting the day at Club de Golf Chapultepec in Mexico City, McIlroy missed a seven-foot birdie chance at the first hole, which set the tone for a frustrating afternoon.

"I needed to get off to a fast start and I didn't. The course got a little tricky over the weekend and maybe I didn't adjust to that," the Northern Irishman told Golf Channel after shooting an even-par 71 to finish equal seventh, four strokes behind winner Dustin Johnson.

A victory would have given McIlroy the world top ranking, provided Johnson had finished worse than solo fourth.

"First start back, it's OK, but being two ahead going into the weekend (it is) obviously disappointing to finish where I have," said the four-times major champion, who had been out for six weeks with a rib injury.

"These two rounds were the sort of rounds I would have expected the first two days, not the last two. I was hoping to improve as the week went on."

American Phil Mickelson also started Sunday two strokes off the pace, but he too shot 71.

But Mickelson said his preparation for the Masters in April was largely on track.

"I hit a lot of good shots," he said. "I know what I'm trying to do and when I do it I'm able to execute and play golf at the level I want, but unfortunately I had a bit of a relapse yesterday."

Third round leader Justin Thomas, meanwhile, faded with a 72 to finish three strokes behind Johnson.

"I got away with some bad shots yesterday and I was swinging it bad again today," Thomas said. "I think it just finally all caught up with me." (Reporting by Andrew Both in Cary, North Carolina, editing by Gene Cherry)