Ponte Vedra Beach - Dustin Johnson got off to a strong start but still has work to do to hang on to his world No 1 ranking as six players shared the lead Thursday at The Players Championship.
Johnson headlined the leading pack on six-under par 66 - his lowest opening round PGA Tour score in 13 months.
Joining Johnson atop the leaderboard at TPC Sawgrass in Ponte Vedra Beach, Florida, were fellow US Open winner Webb Simpson, Chesson Hadley, Matt Kuchar, Patrick Cantlay and in-form Swede Alex Noren.
Johnson, who has been world number one for 64 weeks, is under siege by those ranked from second through fifth, and he answered the challenge with a bogey-free round.
"I definitely want to keep the number one title, but I still have three more days of golf, and I'm going to have to play really well if I want to keep the number one spot," the 33-year-old said.
Kuchar, fuelled by a birdie-eagle-birdie run at the 10th, 11th and 12th, moved to seven-under with birdies at 15 and 16, but a bogey at the last dropped him into the leading group.
A further six players, including defending champion Kim Si-woo of South Korea and were a stroke back on 67.
Kim, the youngest winner of the Players, matched the lowest opening round by a defending champion established by Jack Nicklaus in 1977.
"I liked my score in the first round, but I can hit it better," Kim said.
"I'm fine, good," he said, but added he wasn't yet thinking about a repeat title. "If I was to win again, I feel like it would be dreaming, as that's too hard. This is just the first round. I'm going to keep working hard."
Former Players and Masters champion Sergio Garcia was seven-under with two holes to play, thanks in part to a run of five straight birdies from the eighth through the 12th.
Garcia then undid his then bogey-free round in hitting the woodwork guarding the island green at the famed par-three 17th, his ball disappearing into the water.
A year ago, Garcia had the crowd roaring with delight on day one with a hole-in-one at 17.
On Thursday he walked off with a double-bogey five and then bogeyed the last to finish in a group of 14 players on 68 that also included third-ranked fellow Spaniard Jon Rahm and world number five Justin Rose of England.
Tiger Woods's return to TPC Sawgrass included a 20-foot eagle at the par-five ninth. But he undid a solid display in finding the water off the tee at the last on route to closing bogey and a level par 72.
"I felt like today the way I was hitting the golf ball, especially toward the back nine, that I felt like I should have shot something in the 60s, but didn't do it," Woods said.
Jordan Spieth struggled in posting a shock three-over 75 that included a 12th hole eagle promptly followed by a double-bogey at the very next hole.
Phil Mickelson, playing alongside Woods and Rickie Fowler, ended his round in shocking style.
The 47-year-old was even par after back-to-back birdies at the 11th and 12th but proceeded to post a horror run from the 14th of a double-bogey, bogey and two further double-bogeys, including finding the water at 17, in an eventual seven-over par 79.
It was Mickelson's highest score since a similar first round 79 on route to missing the cut in last August's PGA Championship.
Mickelson blamed fatigue after the strain of fighting his way to a share of fifth in last week's Wells Fargo Championship.
"I knew this was going to be an issue," he said. "I said it Sunday at Wells Fargo, I was worried about energy this week, and I just kind of ran out at the end."
Leading scores after Thursday's first round of the US PGA's The Players Championship at TPC Sawgrass in Ponte Vedra Beach, Florida (par-72, USA unless noted):
66 - Webb Simpson, Dustin Johnson, Alex Noren (SWE), Chesson Hadley, Matt Kuchar, Patrick Cantlay
67 - Steve Stricker, Jhonattan Vegas (VEN), Kim Si-woo (KOR), Keith Mitchell, Rory Sabbatini (RSA), Andrew Landry
68 - Sergio Garcia (ESP), Lucas Glover, Danny Lee (NZL), Charles Howell, Justin Rose (ENG), Bubba Watson, Cody Gribble, C.T. Pan (TPE), Ollie Schniederjans, Billy Horschel, Charl Schwartzel (RSA), Jon Rahm (ESP), Henrik Stenson (SWE), Xander Schauffele