UPDATE 1-Golf-Cool finish helps Clark move two ahead at Colonial

* Clark in position to win first PGA Tour title

* Stricker surrenders overnight lead

* 'Pink Out' at Colonial for breast cancer support

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FORT WORTH, Texas, May 30 (Reuters) - South African Tim Clark, seeking his first PGA Tour title, birdied the par-four last to move two strokes clear of a congested leaderboard in Saturday's third round at the Colonial Invitational.

A runner-up here last year behind Phil Mickelson, Clark coolly rolled in a 19-foot putt on the 18th green for a four-under-par 66 at a sun-soaked Colonial Country Club.

That gave the straight-hitting South African a 17-under total of 193, ahead of Americans Steve Marino (62) and overnight leader Steve Stricker (69), plus Australian Jason Day (65).

Fiji's former world number one Vijay Singh was alone in fifth at 13 under after carding a 69.

"Playing in the last group today, that's going to help me tomorrow to come out and stay somewhat relaxed," Clark told reporters. "I've played great golf and I just need to try and keep that going."