Defending CJ Cup champion Rory McIlroy started strong to be one shot behind joint leaders Gary Woodland and Trey Mullinax after the opening day at Congaree Golf Club in South Carolina.

The Northern Irishman is tied with six players at five-under after carding opening rounds of 66, including recent Shriners Children's Open winner Tom Kim.

McIlroy pieced together birdies on the fourth, fifth and sixth holes, but his round leveled out, finishing bogey-free.

Woodland was one of the pre-tournament favourites and he impressed early, with three straight birdies to open his round.

The American dropped three bogeys to slow his progress, but sunk a 21-foot birdie putt on the par-four 18th to finish his round with nine birdies and a share of the lead.

Co-leader Mullinax birdied four of his first six holes and six of his first 11, highlighted by a 23-foot birdie putt on the 11th.

Kim, who is only 20-years-old, stormed up the leaderboard late with three birdies in his final six holes, including rolling in a 28-foot birdie on the 16th although he slipped from the lead with a bogey on the 18th.

McIlroy and Kim are joined by Cam Davis, Kurt Kitayama, Aaron Wise and Wyndham Clark in carding opening day rounds of five-under-par.

Norwegian Viktor Hovland and American Tyrrell Hatton headline the following six-member group at four-under.

Two-time PGA Championship winner Justin Thomas is back at three-under, while former world number one pair Jon Rahm and Jason Day are at two-under. Scottie Scheffler finished with an even round.

Three-time major winner Jordan Spieth had a poor round that included a double bogey on the sixth and four bogeys to be four-over-par.