The left-hander from Bright's Grove, Ont., led three times during the first 18 holes despite carding four bogeys and three birdies. Casey pulled ahead on the back 18, moving 3-up after 30 holes and sealing the victory when Weir bogeyed the 32nd and 33rd holes.

American Shaun Micheel, meanwhile, might have saved his best golf for Luke Donald.

A day after ending Tiger Woods' five-tournament winning streak, Micheel continued to roll through the championship by burying his English opponent with an onslaught of birdies to build a 5-up lead at the turn of their 36-hole quarter-final.

Micheel won the first two holes with 10-foot birdie putts, and he had six birdies through the first 12 holes to put the 28-year-old Englishman deeper in the hole.

Colin Montgomerie, meanwhile, risked injury with the Ryder Cup one week away by banging a four-iron off a tree on the 17th hole, rattling the club against the trunk while trying to get back to the fairway. It worked, and he halved the hole with Michael Campbell, allowing him to complete a stirring rally that left them all square going to the afternoon.

The other matches were equally close, although no one matched the quality of Micheel's golf.

Robert Karlsson, who shot 64 in his morning round Thursday to dispatch Jim Furyk, was 10 shots higher against Angel Cabrera and still wound up in the lead. Despite being in nine bunkers, Karlsson was 1-up at the turn, taking advantage of sloppy play by the Argentine. Cabrera didn't make his first birdie until the 18th, cutting into his deficit.

The championship offers the richest prize in golf among official events _ with about US$1.87 million going to the winner.