But his opening round of one-over-par 72 left him languishing 10 strokes behind clubhouse leader Marc Leishman in joint 56th.

Leishman shot 10 birdies and dropped just one shot on the sixth on the Chicago course, leading to a career best round of nine-under 62.

McIlroy, who entered the tournament at his lowest world ranking - sixth - for more than three years having missed the cut at the Dell Technologies Championship, got off to a bad start from the first hole, dropping two on the par 4.

While he looked to be improving on the seventh and eight hole, scoring a birdie on each, a bogey on the ninth left him level par at the halfway point.

The back nine saw an even more erratic performance with bogeys on the 12th, 15th and 18th holes.

Englishman Justin Rose fared better with just two bogeys across 18 holes, ending the day with four-under 67.

A standout moment came on the 13th hole when his 21ft birdie putt looked to be going wide only to fall in on the par 4.

Ian Poulter, who played just 13 tournaments last year due to a foot injury, completed the day in the middle of the pack after getting off to a good start with a birdie each on the first two holes.

A bogey on the fifth was the only slip-up in a round that was largely steady for the 41-year-old.

Jordan Spieth, who is still in contention to catch the Australian leader at just three shots adrift, completed the round with a six-under 65, having not dropped a shot all round.

With plenty left to play for before Sunday's final day, 2017 Masters winner Sergio Garcia saw a mixed start to the day, with two birdies and a pair of bogeys before the halfway mark.

Three more birdies followed on the back nine holes, with the Spaniard making par on the rest to complete the day at three-under 68.

AP