Johnnie Walker Championship 2006

Montgomerie, two shots ahead of his playing partner overnight, established an early four-stroke before Casey, winner of the event in 2001, struck back over the back nine.

A seven-under-par 66 by Casey took him to 15-under-par 204, a stroke better than Montgomerie who carded a 69.

Danes Thomas Bjorn, the first round leader (67), and Soren Hansen (68) plus Australian Brett Rumford (67) share third place, three strokes behind Casey.

On the par-five 18th, Casey took two putts from 30 feet to edge ahead of Montgomerie, who, unlike his opponent, was unable to hit the green in two and could manage a par only.

"Colin started brilliantly and I made mistakes on the opening holes," said Casey. "I just knew I had to battle back.

"I do enjoy chasing and that's what I did. Length did work in my favour and the two birdies at the end made the difference in being in the lead or not."

Montgomerie, six days after his U.S. Open bid failed on the last hole at Winged Foot, blamed fatigue and Casey's greater length for relinquishing his lead.

"Paul was hitting 50 yards longer and his length ensured he could get to 16 and 18 while I couldn't," said the Scot. "I'm very, very tired."

Asked whether it was mental tiredness, Montgomerie replied: "Whatever tired you can think of."