Romero, the Argentine star who won the 2006 JELD-WEN Tradition for his lone title on the 50-and-over tour, had a 12-under 132 total on the Sonoma Golf Club course.
'Good concentration, that's the key,' Romero said. 'I never lost the concentration today. Yesterday, I made 64, but sometimes it's hard. Not today. Today was easier.'
Watson shot a 64, and Thorpe, the 2003 and 2006 winner, had a 69. Thorpe earned the last spot in the field, edging Tom Jenkins by $4,079 for 30th place on the money list.
Watson, who earlier this year edged Romero by two strokes to win the Senior PGA Championship in South Carolina, had the day's best round.
Watson began the day five shots behind the leaders and was four shots back after bogeying No. 14. He rebounded nicely, however, closing out the round with four straight birdies to close the gap on Romero.
'My deal this year has been just give me a chance,' said Watson, who missed much of 2006 after surgery on his right shoulder. 'Let me get into contention, let me see if I've still got it. I was kind of cruising (today).'
Watson's only mistake on the back nine was on the 215-yard, par-3 14th when a fan whistled during his backswing. The tee shot landed in a bunker beside the green and Watson wound up with his only bogey of the day.
'I was mad at myself for being affected by the whistle thing, because at this stage in your life in golf, you should be able to hit through anything,' Watson said.
Brad Bryant, one of four players in contention in the Charles Schwab Cup race for a $1 million annuity, shot a 67 to join Tom Purtzer (67) and Joe Ozaki (65) at 9 under. Mark James (70) was 8 under.
Jay Haas, 165 points behind leader Loren Roberts in the Charles Schwab Cup standings, was 6 under after a 70. Roberts was 3 under after a 70. Tom Watson, seeking his fourth victory in the season-ending tournament, was 1 over after a 71.
Watson entered the week 694 points behind Roberts, and Bryant was 697 points back. The tournament winner will receive 880 points.
If Haas and Roberts hold their positions through the remainder of the tournament, Roberts would win the Cup by 17 points.
'I'm putting it bad, hitting it bad,' said Roberts, who tossed his putter at his bag in disgust after missing a short birdie putt on No. 18. 'I just feel like I'm leaving a lot of shots out there. We'll see. Obviously, I'm going to have to play good the next couple of days. I'm running out of time.'