Paul Casey tied the record for the best back-nine in PGA tour history as he catapulted himself into contention on day two of the HP Byron Nelson Championship in Irving, Texas.

The Englishman was one over for the day and struggling to make the cut when he reached the turn but then fired six birdies and an eagle to come home in 27 and sit two shots behind American leader Brendon Todd on a congested leaderboard.

Casey had fired a 71 on Thursday and after three bogeys and an eagle on the front nine, he was two over for the tournament as he stood on the tee at the 435-yard par-four 10th.

But he birdied that and the 11th and picked up further shots on 13,14 and 15 before an eagle on 16 and a closing birdie.

That made him just the eighth player in PGA tour history to shoot 27 over nine holes and his score was just one shot worse than Corey Pavin's front-nine record set at the U.S. Bank Championship in 2006.

"I had no concept of what I was doing," Casey told

"The seal was broken when I got to 10. 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."

Todd, meanwhile, followed up an opening 68 with an impressive 64 to take a two-shot lead over a large group on six under.

The 28-year-old birdied seven, eight, 10, 11 and 13 before dropping his only shot of the day on the par-four 15th.

At that point he was part of a large leading pack on six under but picked up shots on his final two holes to take a two-shot lead into the weekend.

Last week's Players Championship winner Martin Kaymer was among the chasing pack alongside Casey, Canadian duo Graham DeLaet and Mike Weir, Australian Marc Leishman and Americans Morgan Hoffman, Tim Herron and Charles Howell.

Two-time Open champion Padraig Harrington was two shots further back after a second consecutive 68 alongside American Matt Kuchar and South Africa's Louis Oosthuizen.

In-form Jordan Spieth was in the group at three under, one shot clear of overnight leader Peter Hanson who fired a disappointing 73.

Another former Open champion, David Duval, was just one shot off the lead after the first day but was suffering with an elbow problem and missed the cut by one shot after slipping to a 76 on Friday.

Asked if the injury had impacted on his performance, Duval said: "It did. To what degree, I don't know."