(CNN) -- Tiger Woods endured a torrid time during the opening round of the Bridgestone Invitational as he battles to cling onto his world number one ranking.
Woods recorded his worst ever round at Firestone Country Club in Ohio -- despite winning the tournament seven times in his last ten attempts -- shooting a four-over-par 74 to finish a full ten shots behind leader Bubba Watson.
Fellow American Phil Mickelson is hot on Woods' tail and can claim the title of world's best golfer if he wins or finishes in the top four and his rival finishes 38th.
Englishman Lee Westwood can also grab the top spot if he wins the tournament and Woods comes outside the top two, but he had a disappointing opening round of 71 to finish one-over-par.
Woods made three bogeys in the opening five holes and was particularly wayward off the tee during his round.
Further bogeys at holes nine, 11 and 14 were tempered by birdies on the sixth and 17th as he ended two shots past his previous worst round at Firestone.
Mickelson is second behind Watson on four-under-par with Kenny Perry and Australian Adam Scott. U.S. Open winner Graeme McDowell joined them after he finished with four straight birdies.
Mickelson's round was ignited by a 50-feet chip-in to make birdie on the sixth hole. "I pulled an iron shot to the right, and I needed to get that up and down to maintain some momentum," Mickelson told the PGA Tour website.
"I was only one under par, and I was just trying to keep it just a couple under, seeing if I could get one or two more. And when that lob shot went in the hole, it gave me a nice boost.
"Not only did I get it up and down but I made a nice birdie and was able to get two more coming in."
But it was Bubba Watson who carded a superb six-under-par 64 on his first appearance at the Bridgestone Invitational to lead the pack.
Watson birdied the first and fifth holes before four consecutive birdies on the back nine helped him to grab the lead.
He told the PGA Tour website: "It's tough to make shots around this golf course. It's a shotmaker's golf course. But it's in beautiful shape. It's probably one of the best shaped golf courses all year.
"I knew it was going to be tough, but today I just putted really well, hit good iron shots and made it look a little bit easy, but we're not done yet. We've still got three tough days of golf to go."