Allen was among seven players who shared the first-round lead at 3-under. He birdied five of the first eight holes and eagled the par-5 14th while shooting his best round of the year in hot, breezy conditions at the par-70 Omaha Country Club.
Allen’s 36-hole lead is the largest in the event’s 34-year history.
Rocco Mediate, at 5-under, is his closest pursuer. Mediate finished with 14 consecutive pars while shooting a bogey-free 67. Jeff Sluman, who also shot 67, is at 4-under.
Allen was short with his birdie putt on the last hole, ending his bid to match the U.S. Senior Open record of 62 by Loren Roberts at Prairie Dunes in 2006.
The 54-year-old Allen has four victories since joining the Champions Tour four years ago, including the 2009 Senior PGA Championship.
WEB.COM TOUR: In Sandy, Utah, Chad Collins missed a chance for the fourth 59 in tour history, parring the final six holes for 11-under-par 60 in the Utah Championship.
The 34-year-old Collins missed a 15-foot uphill birdie putt on the par-4 ninth, his last hole in the second round at Willow Creek Country Club that would have given him 59.
He had an eagle and nine birdies on the first 12 holes and matched the tour’s nine-hole record with 27.
Aiken’s Kevin Kisner (71-66–137) and Augusta’s Scott Parel (70-69–139) missed the cut, which fell at a blistering 6-under 136,
EUROPEAN TOUR: In Inverness, Scotland, Ernie Els and Phil Mickelson endured frustrating second rounds at the Scottish Open as the tournament’s headline acts were upstaged by some lesser-knowns.
Chris Doak, ranked No. 341, was the surprise leader at the halfway point after a second consecutive 6-under-par 66 left him on 12-under 132, while Matthew Southgate (64) was in a tie for second place with Ross Fisher (65), Peter Uihlein (66) and JB Hansen (65).
Conditions were ripe for low scoring on the Castle Stuart links but Els struggled on the greens and shot 72-70 to miss the cut by two shots a week before he defends his British Open title.
“I had no idea on these greens – I’m glad I’m leaving,” Els said.
Mickelson, another four-time major winner, was tied for third after a first-round 66 but dropped after 70.
Former Augusta State golfer Scott Jamieson missed the cut after rounds of 70-80.
LPGA TOUR: In Waterloo, Ontario, Catriona Matthew shot 7-under-par 64 to take a three-stroke lead after the second round of the Manulife Financial LPGA Classic.
Matthew is at 15-under 127 through two rounds.
Angela Stanford shot 67 and was second at 130. Five players are at 131. Inbee Park, who is looking to win her fourth consecutive win, had 67 and is at 132.