Rory McIlroy has regained top spot in the world rankings after a two shot win at the WGC-Bridgestone Invitational in Ohio.

The Northern Irishman was trailing Sergio Garcia by three shots heading into the final round at the Firestone Country Club on Sunday but chased down the Spaniard's lead and eventually overcame it.

He finished with a 66, taking him to -15 for the tournament, while Garcia slipped out of contention after shooting a 71, making him -13.

McIlroy not only picked up the winner's cheque of $1,530,000, but toppled Australian Adam Scott as number one in the world.

The 25-year-old has now achieved back-to-back victories after the Open Championship, becoming the first player since Tiger Woods in 2006 to follow up a major win with a victory at his next start.

The three-time major winner put his success down to his driving. "Whenever I drive the ball well, I always put myself in positions where I can attack flags and try and make birdies, but when I'm swinging it well with a driver, that sort of funnels through the rest of my game," McIlroy said.

"If I'm swinging well with a driver, more than likely, I'm swinging well with everything else."

There was less good news for last year's winner Tiger Woods, who was forced to pull out of the Bridgestone Invitational after aggravating a back injury.

Woods walked off the course at the ninth hole on Sunday after sending a tee shot wayward, casting doubt on his fitness ahead of next month's Ryder Cup at Gleneagles, Scotland.

"I fell back into the bunker," he said. "It just jarred it, and it's been spasming ever since. It's just the whole lower back. I'm not sure what happened."