Hunter Mahan finished with a flurry of birdies in an eight-under-par 64 to seize the halfway lead in the RBC Canadian Open.
His fellow American John Merrick had set the clubhouse lead on 11-under 133 after a magical 10-under 62 that contained an eagle and eight birdies, but Mahan jumped into a two-shot advantage after following up his opening 67 in style.
The 31-year-old Mahan had a ninth-placed finish at the Open Championship last week and the five-time winner on the PGA Tour gave himself another excellent chance to contend for silverware this weekend.
He birdied two, three, four, 11 and 13 before finishing with gains at each of the last three holes.
On a leaderboard dominated by players from the United States, Bubba Watson occupied third position on nine under after bettering his opening 68 by one shot, while a pair of 68s have put Patrick Reed on eight under. Australian Aaron Baddeley has also had two rounds of 68 so far.
Tommy Gainey jumped 87 places after a huge improvement on his first-round 73, managing an eight-under 64 second time around to join Chris Kirk in a share of sixth place on seven under.
The overnight leader Brendan Steele had a dismal day. Yesterday's 65 put him at the front of the pack, and a 75 knocked him back to four under.
But Northern Ireland's Graeme McDowell put a dismal Thursday behind him as he improved on his opening round by 11 shots, banishing memories of his 76 by going round in a sparkling 65 that included an eagle at the long 16th hole.
He is staying for the weekend, and more of the same could hoist the former US Open champion into contention.
Former world number one Luke Donald missed the cut by one shot after a one-under 71 put the Englishman on level par for the tournament.