SYLVIS, United States (AFP) - Reigning Masters and US Open champion Jordan Spieth ratcheted up his preparation for next week's British Open by firing a seven-under 64 in Friday's second round of the PGA John Deere Classic.

While fellow American Justin Thomas followed a 63 with a 67 to seize the overall lead, attention focused on Spieth as he bids to win the first three majors of the year.

World number two Spieth found 10 of 14 fairways and reached 13 of 18 greens in regulation in making six birdies and an eagle against a lone bogey to move into a share of 16th after an opening 71.

"Today was a big day," he said. "I made a huge step forward today based on how I felt yesterday. I wasn't feeling good about my game. Ultimately, this was a big day for me in my preparation for next week."

Spieth hopes to match Ben Hogan as the only player to win the first three majors of the season in the same year with a victory next week at St Andrews.

The feat would give him the chance to complete a calendar year sweep of the majors at next month's PGA Championship.