Garcia, at 15-under 198 for the tournament, birdied the 16th and 18th holes at the 6,917-yard, par-71 Phoenix Country Club. Thomas Bjrn, one stroke back after two rounds, is second at 12-under 201 heading into Sunday's final round.
After a slow start in which he missed a five-foot birdie putt on the first hole, Garcia turned his game around with a curling 20-foot birdie putt on the fifth hole.
'I hit a great putt on 4 and on 5, I hit another great putt and it went in,' said the 23-year-old Spaniard, who has finished runner-up here twice. 'I know how good a player Thomas is, and I can't let up for a minute.'
Bjrn was just one stroke back of Garcia after both players birdied the par-3 16th, but a bogey on the final hole left him three strokes of the pace.
'It was a disappointing finish,' Bjrn said. 'It just makes tomorrow that much more difficult. Sergio is a good player and he knows how to win. If you don't play your best against the best, you've got no chance.'
Japan's Toshimitsu Izawa fired a 67 and is five strokes off the lead.
American Hank Kuehne, who won last week's Franklin Templeton Shootout title with partner Jeff Sluman, had the day's best round of 7-under 64 to move up the leaderboard into fifth place.
Kuehne's approach on the 18th found the greenside bunker, but he chipped in from the trap for an eagle.
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