The defending champion Padraig Harrington had to produce a five under par 67 just to remain four behind David Toms at the halfway stage of the Honda Classic here.
The title favourite and joint first day leader Toms had an eagle and seven birdies, but after threatening to run away with the event midway through the second round, he gave the chasing pack hope with three bogeys in the last five holes to stand at 10 under.
Playing partner Harrington, out in 32 after four successive birdies, picked up more shots on the 12th and 17th, but the second of those widened his deficit as Toms holed from off the green for eagle.
Harrington, lying joint third as play came to a close, is one of five Europeans who go into the weekend with hopes of toppling Toms.
Swedes Daniel Chopra and Mathias Gronberg are also on six under, Chopra after a 65 and Gronberg following a 71, while Lee Westwood and Luke Donald are one further back.
Three weeks after finishing fifth in the Nissan Open in Los Angeles Westwood added a 66 to his opening 73 and from joint 47th overnight even found himself out in front for a while.
Donald, meanwhile, played his last seven holes in a spectacular six under par for a 67.
Westwood, seeking his first victory since 2003, turned in 33 with birdies at the 10th, 12th and 14th and then had three more in a row from the fifth.
That was good, but what Donald did over the same stretch was even better. The world No 14, second favourite for the title based on the rankings, followed birdies at the third, fourth and fifth with a 20-foot eagle putt on the next and then picked up another shot at the short eighth to be home in 30.
Toms will resume today one ahead of fellow American Billy Mayfair, while another in the group on six under is Australian Geoff Ogilvy, winner of the Accenture World Match Play title two weeks ago.