Defending champion Thompson signed for four under 67 around TPC Deere Run on Saturday, enough to move him to 15 under for the tournament, one stroke ahead of a chasing pack featuring fellow Americans Max Homa, David Lipsky, Bryan Campbell and Argentine Emiliano Grillo.
The American made the turn at even par but rallied on the back nine to roll in four birdies. It could have been even better, as he missed an eight-foot eagle attempt on the par five 17th hole. Thompson's ten-foot birdie putt on the 18th hole broke him free of a logjam that at one point featured no less than players tied for the lead.
"Nice to have some momentum going into tomorrow," the 26-year-old told AP.
Lurking one shot behind is Homa, whose had a difficult time of it of late. The former world number five has been in poor form while changing coaches, equipment and twice changing caddies.
"Golf has just been very boring for me this year," quipped the world number 99.
"I haven't had a whole lot of stress, and you want to be stressed out. So I look forward to the butterflies in the morning; I look forward to the first tee shot. It's just nice to get to feel that again. It's been a while."
36-hole leader Doug Ghim played his last 10 holes with two bogeys and no birdies and shot 74, pushing him outside the top 20.
Lucas Glover and Zac Blair share the first-round lead at the John Deere Classic after both carded seven-under-par 63s, while hometown favourite Zach Johnson sits just one shot back following an impressive finish.
Viktor Hovland has a one-stroke lead over Scottie Scheffler heading into the final round of the Travelers Championship on Sunday.
Scottie Scheffler came within inches of making PGA Tour history on Friday, carding a brilliant 10-under-par 60 to take the clubhouse lead at the Travelers Championship.
Eric Cole has a one-shot lead at the top of the Travelers Championship leaderboard after the opening round, but Scottie Scheffler lurks in the chasing group.
