World number three Rory McIlroy overcame a stomach complaint to make an excellent start to the WGC-Mexico Championship on his return to competitive action.
McIlroy had played just once in 2017 due to a rib injury which saw him push through the pain barrier before losing to Graeme Storm in a play-off for the South African Open in January.
The 27-year-old still arrived at Chapultepec Golf Club knowing that he could reclaim top spot in the world rankings for the first time since August 2015 by winning his third World Golf Championship title, as long as current number one Dustin Johnson finishes joint fourth or worse.
And the four-time major winner was pleased with an opening three-under-par 68 which left him just a shot off the clubhouse lead shared by Lee Westwood, Ross Fisher, Phil Mickelson, Jimmy Walker, Jon Rahm and Ryan Moore.
Starting on the back nine of a course which reaches a maximum of 7,800 feet above sea level, McIlroy holed from 15 feet for birdie on the par-five 11th and added seven pars in succession to reach the turn in 35.
The four-time major winner then drove into a greenside bunker on the short par-four first hole and got up and down for birdie to reach two under par, while Johnson ran up a double bogey after hitting his tee shot out of bounds.
McIlroy dropped his first shot of the day by three-putting the fifth from long range, but produced the ideal response by holing from 30 feet for an eagle on the par-five sixth. Johnson birdied two of his last four holes to finish one under.
In quotes reported by the Daily Telegraph, McIlory said he was not sure what had caused his stomach problem, adding: "I ate with Erica [Stoll, his fiancee] and my parents and they seemed fine this morning. But I haven't eaten so I'm a little weak."
He added in an interview on Sky Sports: "The rib is fine, it's great actually. How I've responded the last couple of weeks, ramping up the practice and then playing my first full competitive round today, I didn't feel it at all, so it's all positive in that regard.
"I probably could have played the Honda (Classic) last week but I wasn't going to know how my body was going to react playing four days in a row.
"I'm taking next week off to see how everything reacts and responds so it was the logical place to come back. I stayed patient and the week longer will definitely pay off."
Westwood, who is seeking a first WGC title at the 56th attempt, enjoyed a two-shot lead after playing his first 13 holes in six under par, but bogeyed three of the last five.
In contrast, Mickelson birdied the 15th and 18th in pursuit of his first victory since claiming a fifth major title in the 2013 Open Championship at Muirfield.
"This is an incredible challenge for all of us because we're dealing with a great mixture of holes, but more than that dealing with distance control," the 46-year-old left-hander said.
"All the guys out here are so great at distance control that you are always seeing them pin high, but it's a real challenge to do that here because of the difference in how far the ball goes."
Open champion Henrik Stenson withdrew with a reported stomach virus midway through his opening round.