Darron Stiles, playing in the last group on the course, birdied the 18th to tie Janzen for the lead.
Janzen, who will turn 45 next month, shot a 7-under 64 at the TPC-Deere Run course despite suffering a bogey on the par-4 18th. He put his second shot on the 476-yard hole into a greenside bunker and eventually missed a seven-foot par putt.
Thursday's score was Janzen's lowest since shooting a 64 at the Honda Classic in 2005.
This is the 11th event of the year for Stiles and he has survived the 36-hole cut only once.
J.J. Henry, Matt Bettencourt and Dean Wilson were tied for third at 65. Dicky Pride, Jason Day, Tim Petrovic and Matthew Borchert were another shot further back.
Defending champion Kenny Perry, making his first appearance since winning the Travelers Championship two weeks ago, shot a 68 and was tied for 28th place.
Most of the tour's top names are absent while preparing for next week's British Open.