Allen made an 8-foot putt to save par on the final hole Friday after hitting his approach between the grandstands surrounding the green, preserving a 2-under 69 for a share of the second-round lead with Fred Couples and David Frost.
'Got a little lucky,' said Allen, second in the event last year. 'I was trying to punch a 6-iron out to the right, which took off and went back into the stands. I made a great drop, hit a nice shot (then) knocked that one in.'
The 18th hole is one of the signature holes at the coastside course and is playing tough this week. Only two players broke par on the 440-yard, par-4 hole in the second round, while 11 settled for bogey, making Allen's par save critical.
Couples, tied for the first-round lead with Jay Haas after a 68, rallied for a 70 after a double bogey on the opening hole. Frost had a 69 to match Couples and Allen at 4 under at TPC Harding Park.
Charles Schwab Cup points leader Tom Lehman had a 72 to fall four strokes behind the leaders.
Allen hasn't won on the tour since the Senior PGA Championship in May 2009 – the first tournament he played in on the 50-and-over circuit. He's had a handful of top-five finishes since then, including last year at Harding Park when he was two strokes behind John Cook.
The Bay Area native is back in contention again this time despite sputtering on the back nine with bogeys on Nos. 11 and 12. That briefly dropped Allen two shots behind the leaders, but he made up for it with birdies on 14 and 16, then made his nice save for par on the 18th.
It wasn't easy.
Allen pushed his 6-iron approach shot wide right and the ball landed in a tight gap between the grandstands. After taking a drop, Allen hit a chip shot that settled softly on the green before making his par putt. That brought a loud roar from the crowd, including a large group of Allen's supporters who followed him throughout the round.
'It starts with the course,' Allen said about his success at Harding. 'I can see the breaks a little better, so it's comfortable for me. It used to be, growing up out here, bumps everywhere. You had to have some nerve to putt them.'
Six players held at least a share of the lead before Frost briefly pulled away.
Frost, winless on the tour this season, made one of his best shots on the 480-yard par-4 12th when he chipped in for birdie from 12 yards out. That got the South African to 6 under but bogeys on 13 and 17 dropped him back to the pack in the clear-but-chilly conditions.
'It was just hard shaping the ball when it's so cold,' Frost said. 'My left-hand grip has been a little weak on the club, which has not enabled me to come back around with the club. I played better the last 10 holes.'
Couples, a two-time winner on tour this year, three-putted the par-4 first, then made eight consecutive pars before three birdies on the back nine gave him a share of the lead. The U.S. Presidents Cup captain is trying to become the first back-to-back winner on tour this year.
Haas aggravated a lower-back injury midway through the round. Haas walked gingerly and winced noticeably over the final seven holes, picking up a double bogey on No. 12 and a bogey on 13 to fall back.
Calcavecchia had an erratic day. He holed out for an eagle on the par-4 seventh and had four birdies, but also had three bogeys. He needs to move up at least two more spots on the leaderboard to have a chance at passing Lehman for the season points title.
'I'm in a good spot,' said Calcavecchia, who played with a new driver after Ping shipped him one overnight. 'I just made no putts today. The good news is I holed out a wedge and had a couple close tap-in birdies.'