Zach Johnson, Rory Sabbatini and Brian Harman were in a three-way tie for the lead after firing eight-under-par 63s in the first round of the John Deere Classic in Illinois on Thursday.

Johnson, the 2007 Masters champion, birdied eight of his first 11 holes after starting on the 10th at TPC Deere Run. South African Sabbatini also enjoyed a bogey-free opening round, while Harman's only blemish was a dropped shot at the par-five 15th that marred a run of six birdies in seven holes from the 11th.

Todd Hamilton, William McGirt and Australia's Steven Bowditch were on seven under after 64s, with Zimbabwe's Brendon de Jonge tied for seventh a further shot back along with David Toms, Kevin Tway and Robert Streb.

Three-time champion Steve Stricker had to settle for a three-under 68, while defending champion Jordan Spieth needed birdies on 16 and 17 to salvage a level-par 71.

Johnson, defeated in a play-off by Spieth as defending champion last year, admitted thoughts of a sub-60 round had entered his head "very briefly" after his blistering start.

"I don't want to say I should have shot 61 or 58 or 60 or 59 or whatever," he added on "But I had a chance."

The world number 16 has not managed a top-20 finish in five starts since mid-May, but was pleased to be back in good touch after a two-week break.

"I have found myself a little impatient in the weeks prior to this week," he said. "In Hartford, I missed the cut by 35 shots, it felt like.

"In the US Open, I made the cut, but it was pretty boring, bad golf after that. I was a little spent."