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Defending Masters champion Phil Mickelson sent a warning of intent ahead of next week's Major with a three-shot victory at the Houston Open.

The left-hander carded a final-round, seven-under 65 to finish on -20, with US compatriot Scott Verplank and rookie Chris Kirk in joint second.

Joint overnight leader Mickelson put aside two early bogeys with five birdies on the back nine.

The victory moves Mickelson to third in the world rankings above Tiger Woods.

It is the first time since 1997 that the 40-year-old has been ranked higher than Woods, who drops two places to seventh.

After firing a superb nine-under 63 on Saturday, which propelled him to the top of the leaderboard, Mickelson began his final round with two successive bogeys at the par-four second and third holes.

The two dropped shots allowed fellow joint overnight leader Verplank the opportunity to move ahead, but five successive birdies from the ninth to 13th holes saw Mickelson overtake his playing partner.

Verplank's challenge finished on the 16th when his tee shot landed into the greenside bunker before two-putting for bogey, three shots adrift of Mickelson, two holed two final pars to seal the victory.

The three-time Masters champion said he must concentrate on his mental game to successfully retain his title at the prestigious Masters, which starts next Thursday.

"I'm very pleased to have played well," he said. "I'm just trying to get my game ready and it's not just the physical ball-striking and short game and putting and so forth."

"I've got to maintain my focus throughout the round. There were two points out there where I just lost focus. Next week at Augusta, that golf course can be very penalising."

Houston is a significant location for Mickelson, whose wife and mother were both treated for breast cancer in the Texan city.

The tournament victory also takes Mickelson's PGA career tally to 39, tying Tom Watson and Cary Middlecoff for ninth on the all-time list.

Rookie Kirk, who topped the leaderboard after the second round, earned his first top-two finish on the PGA Tour.

Ireland's Padraig Harrington finished in jointh eighth place at -11 while world number two Lee Westwood finished five shots adrift of his Ryder Cup team-mate.

Englishman Westwood needed a top-two finish to move above Martin Kaymer in the world rankings, but could only manage 30th place.