Dallas - Britain's Paul Casey fired a nine-under 27 on the back nine on Friday to surge into contention at the PGA Tour's Byron Nelson Championship behind halfway leader Brendon Todd.
Todd, a graduate of the Web.com developmental tour, had seven birdies in his six-under par second-round 64 for a 36-hole total of eight-under par 132.
The event has a purse of $6.9 million.
Todd was two strokes in front of a group of eight players headed by Casey, whose seven-under 63 - the low round of the day - came courtesy of his scintillating back nine.
Casey became only the eighth player in PGA history to fire 27 for nine holes, one off the tour record 26 fired by American Corey Pavin on the front nine in the first round at Milwaukee in 2006.
"I had no concept of what I was doing," Casey said after completing 36 holes at six-under 134.
Casey's lone US PGA title came at the 2009 Houston Open, but he has 13 other global triumphs, including last year's Irish Open.
Casey took bogeys at the third and par-3 fifth holes and although he pulled one back with a birdie at the par-5 seventh he closed the front nine with another bogey.
Then came his blazing back nine.
Casey sank a six-foot birdie putt at the 10th and followed with a 24-foot birdie putt at 11.
After finding a greenside bunker at 12, Casey saved par from 13 feet and followed with a birdie from 13 feet at the par-3 13th hole.
Casey added birdie putts from nine feet at 14 and four feet at 15 before making an eagle at the par-5 16th, dropping his second shot 32 feet from the cup and then dropping in the putt.
He followed by parring the par-3 17th and closing with a birdie, sinking a five-foot putt at 18.
"The seal was broken when I got to 10," Casey said. "It just spiraled from there."
"I like to think I would be pretty good at shooting low rounds of golf through the past, but certainly nothing like that."
Casey's prior low score over nine holes in a US PGA event was a 30, which he achieved twice, most recently at New Orleans last month.
But Casey is just one of many players within striking distance of Todd, sharing second place with Germany's Martin Kaymer, former Masters champion Mike Weir and Weir's fellow Canadian Graham DeLaet, Australian Marc Leishman and Americans Morgan Hoffman, Tim Herron and Charles Howell.
Kaymer, winner of last week's prestigious Players Championship, posted his second straight 67. Weir, Howell, DeLaet, Hoffmann and Herron all carded 66s while Leishman posted a 68.
Another five players, South Africans Retief Goosen and Tyrone Van Aswegan and Americans Gary Woodland, Ryan Palmer and Boo Weekley, were tied for 10th on 135.
Todd separated himself atop the crowded leaderboard with birdies at 17 and 18, capping a round in which he needed only 22 putts.
"It's nice to put two under-par rounds together before the cut," Todd said. "That's been a challenge for me this year. I'm excited to be in the last group on the weekend."
Leading second-round scores on Friday in the US PGA Tour $6.9 million Byron Nelson Championship (USA unless noted, par-70):
132 - Brendon Todd 68-64
134 - Graham DeLaet (CAN) 68-66, Morgan Hoffmann 68-66, Martin Kaymer (GER) 67-67, Mike Weir (CAN) 68-66, Paul Casey (ENG) 71-63, Tim Herron 68-66, Marc Leishman (AUS) 66-68, Charles Howell 68-66
135 - Gary Woodland 68-67, Retief Goosen (RSA) 70-65, Ryan Palmer 67-68, Boo Weekley 67-68, Tyrone Van Aswegen (RSA) 67-68
136 - James Hahn 71-65, Matt Kuchar 69-67, Padraig Harrington (IRL) 68-68, Louis Oosthuizen (RSA) 68-68
137 - Alex Cejka (GER) 67-70, Charlie Beljan 72-65, Tim Wilkinson (NZL) 66-71, Andres Romero (ARG) 71-66, Vijay Singh (FIJ) 69-68, Jordan Spieth 70-67