The 21-year-old, who has opted to play in the PGA Tour event this week rather than the Scottish Open ahead of next week???s 144th Open, was well off the pace following a level-par 71 in the opening round in Silvis, Illinois.
However, the Masters and US Open champion, playing his first tournament in three weeks, mixed an eagle at the par-five second with six birdies and a lone bogey to move to seven-under 135 at halfway and within sight of the leaders.
After teeing off at the 10th he birdied the 14th and 16th before adding back-to-back birdies at the 18th and first before his eagle at the second after holing a 10-foot putt.
Further birdies followed at the fifth and seventh before he dropped his only shot at the eighth.
Spieth, who took time off after his US Open win at Chambers Bay last month, was delighted with his round – he found 10 of 14 fairways and reached 13 of 18 greens in regulation – after feeling a little rusty on Thursday.
He said: “Today was a big step. This was the third round of golf I’ve played since the US Open. With the pro-am on Wednesday being the first full round and then yesterday, it’s just repetitions. I’m getting on-course reps and it’s making a difference.
“I’m working on some things coming off a little break there, and my putter. I was a little more aggressive with it today and it paid off. Today was a solid round to get myself in a position to make a run at it (winning the tournament).”
Spieth will travel to St Andrews next week hoping to match Ben Hogan as the only player to win the first three majors of the season in the same year, a feat that would give him the chance to complete a calendar year sweep of the majors at next month’s PGA Championship.
He added on Sky Sports 4:??”Next week is on my mind. I’m really solely focused on how I can get the third leg right now because it is a career goal of mine to have a career grand slam. So if it happens in one year that would be something – that would be pretty special.”