World number one Dustin Johnson says he is back to his best after beating Jordan Spieth in a play-off at the Northern Trust Open.

Johnson started the season in blistering fashion before injuring his back in a fall on the eve of the Masters in April, saying it has taken him this long to get back to where he was.

Johnson, who was five shots behind Spieth at one point, forced a play-off with his fellow American after sinking a 17-foot putt on the last to also finish on 13 under par.

Johnson then won the first sudden-death hole and speaking afterwards said he is delighted to be back at the top of his game.

"I feel the game's back in form like it was leading into the Masters," he said. "I'm swinging really well, got a lot of control over the golf ball and I'm feeling really good.

"Getting a win here gives me a lot of confidence going into next week and the rest of the play-offs."

Speaking afterwards Open Champion Spieth said: "I didn't lose this tournament. He won it."