Izawa makes early running with six-under par 65
Izawa holds a two-stroke lead over England's Justin Rose and a three-shot margin over Fred Funk and Denmark's Thomas Bjorn.
Izawa, who started his round on the 10th tee, picked up two birdies in the first nine holes to make the turn at two-under.
He had five birdies and a lone bogey on his final nine for the 65 at Sahalee Country Club.
World number one Tiger Woods and PGA champion Rich Beem were among the later starters.
Ireland's Darren Clarke got off to a dream start as he was four under after four holes to share the lead with three others. But the Dungannon man came back to the field, though he rallied on the closing holes to finish at three under, shooting six birdies, six pars and three bogeys.
Out of form Lee Westwood missed a glorious chance to take the outright lead at the start of the Seattle event.
After a 15-minute fog delay Westwood, trying to fight his way out of a slump that has sent him crashing from fourth in the world to his current 144th, opened with a 12-foot birdie putt.
He was then on in two at the long 11th his second but three-putted for par, missing a four-footer.
Westwood made another 12-footer at the 12th and was only one behind.
Still, the early signs were that this was a far improved Westwood from the man who shot a season's-worst 82 to miss the halfway cut in the US PGA championship last week.
He holed a 15-footer at the 15th to lead on his own for the first time in any tournament for ages.
Westwood's fortunes changed at the start of the outward half. He bogeyed both the first and the 509-yard second to go back to two under.
Padraig Harrington birdied the long 11th to grab a share of top spot, but dropped back to finish one over par (72).
Rose, playing in his first world championship, began brightly with a birdie at the 406-yard first and remained one under after five.
However, Scot Paul Lawrie double-bogeyed the 509-yard second and dropped another shot at the next to be in last place at three over.
Colin Montgomerie missed the cut last week and made only one birdie in 36 holes. But when he made putts of 14 and 30 feet at the 14th and 15th he had already had three in six holes yesterday and shared the lead with American Scott McCarron.
Montgomerie was in trouble off the tee at the 16th and after his second shot went less than 20 yards he did well to drop only one stroke.
Garcia and Izawa were joined in second place by Jose Maria Olazabal, while Woods hit his approach close on the seventh and improved to three under.
Garcia was in trouble at the 10th, driving left and then hitting his second into water for a double bogey six.
He did birdie the long 11th, however, to climb back to three under alongside Olazabal, who bogeyed the first after turning in 32.
Izawa was the new leader, birdies at the fifth and seventh making him five under, one ahead of Woods, who birdied the 10th, and Justin Leonard, anxious to make up for losing the last round lead last week.
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