Craig Bowden posted a 6-under 66 for third place at the TPC at Sugarloaf.
Reigning PGA Champion Shaun Micheel and Tim Petrovic recorded matching rounds of 5-under 67 for fourth place.
Olazabal broke out of the gate quickly with a 20-foot birdie putt at the second hole. Sharp iron play led to tap-in birdies at five and eight for the two-time Masters champion.
At the par-five 10th, Olazabal missed the green but was able to two-putt for another birdie. The Spaniard again knocked an approach to tap-in range at the 12th with a 7-iron and was two off the lead.
Olazabal hit a 5-iron to eight feet at 15 and drained the birdie putt to reach 6-under par. He laid up with a 3-iron at No. 18 and wedged his approach to three feet to set up the birdie that tied him atop the leaderboard with Tambellini.
'It was an extremely good round for me today,' said Olazabal. 'I put the ball pretty close most of the time, and the driver didn't hurt me that much. I missed a few drives but they didn't hurt me all that much.'
Olazabal has struggled in the 2004 PGA Tour campaign as his best finish was a tie for 32nd place at the Nissan Open. With the Masters looming next week, the Spaniard hopes a good week this week can help him at Augusta National.
'I've always said this golf course is set up pretty much the same way as Augusta,' said Olazabal. 'You do have very undulating greens with a lot of slopes around the green. There's not much rough around the greens. It's a good tune up for next week.'
Olazabal has 28 wins worldwide while his co-leader, a winner on the Nationwide Tour last year, is making just his seventh career start on the PGA Tour.
Tambellini opened on the back nine Thursday. Early on, his round did not look like a 65. He hit his approach long at the 12th and missed a 20-footer to save par. The 29-year-old Californian didn't make another mistake.
At the 14th, Tambellini knocked a 6-iron to four feet to set up his first birdie of the round. Three holes later, it was a wedge to 20 feet for his second birdie and at 18, Tambellini reached the green in two shots and lipped out a 25-foot eagle putt before a tap-in birdie.
Tambellini parred the first three holes of his second nine. Then, at the par-5 fourth, he drove into a fairway bunker and wedged his second into the fairway. After hitting a poor 3-iron from 196 yards, Tambellini still holed the putt for birdie, triggering an excellent run to close his round.
Tambellini drove into the bunkers at five and wedged his second short of the water. He hit another wedge to five feet and drained the putt to save par.
'That was a key hole, really,' said Tambellini. 'That really kept my round going.'
Tambellini missed the green with a 3-wood for his second shot at the par-5 sixth. He holed out his bunker shot for eagle, then tapped in for birdie at seven and sank a 25-footer for birdie at the eighth to polish off his round of 65.
'It's not something that I would have ever dreamed of doing this morning,' said Tambellini. 'My putting stroke didn't play great. A round like this, you don't pick them, they pick you.'
Adam Scott, the winner of last week's Players Championship, opened with a 4-under 68. He is tied for sixth place with defending champion Ben Crane, Billy Mayfair, Brian Gay, Rory Sabbatini and former U.S. Open champion Scott Simpson.
Phil Mickelson, the 2000 BellSouth Classic champion, is tied for 12th at 3-under-par 69. Former PGA Championship winner Rich Beem is among those tied with Mickelson.