Garcia holed two shots from off the green and nearly holed a third at the last for a round of 5-under-par 66.
The 21-year-old Spaniard leads Scott Hoch (68) by two shots at 12-under-par entering Mondays final round of the Buick Classic.
A series of rain delays throughout the week forced the extra day of play in Harrison, N.Y. Sundays third round was the first round to be played uninterrupted.
1998 Buick Classic champion J.P. Hayes is in third place at 9-under-par following a 67. Russ Cochran (67), Gabriel Hjertstedt (68) and Stewart Cink (69) are all tied for fourth place at 7-under.
Tiger Woods shot 68 on Saturday at Westchester Country Club to finish the third round eight shots off the lead at 4-under-par.
Click for a virtual tour of Westchester C.C.
Woods is still feeling the effects of an opening 75. Saturday, he made the turn in 4-under-par 32, but recorded two bogeys to just one birdie on the back nine.
The largest third-round deficit Woods has ever made up to win on the PGA Tour is five shots in the 2000 AT&T Pebble Beach National Pro-Am.
'I'm going to have to to play a great round,' Woods said. 'When you're this far behind, you're going to need a little help. The way the guys on top of the board are playing, it's going to be awfully tough to catch them.'
'I'm more focused on Scott Hoch and J.P. Hayes (than Woods),' he said.
Garcia, who collected his first tour victory at this years MasterCard Colonial, nearly won this event a year ago. He led in the final round, but carded a double bogey on the 11th to fall from contention.
Saturday, Garcia birdied the 5th and 6th hole and then nearly drove the green on the short par-4 7th. From off the green, he flopped a wedge shot that landed about five feet short of the hole and rolled in for an eagle-2.
Garcia went on to birdie the 9th, 17th and 18th to finish strongly after bogeying the 12th and 16th holes.
Garcias bogey at the 16th cut his lead over Hoch to just one. Hoch then tied for the top spot with a chip-in birdie at the par-4 17th. However, Garcia quickly responded with a chip-in birdie of his own to reclaim a one-shot lead entering the par-5 18th.
At the 18th, Garcia played his second shot behind the green, from where he rolled his pitch shot along the edge of the cup. He tapped in for birdie and a two-shot advantage.
Full-field scores from the Buick Classic