Thorpe (69) and Fleisher (65) are at 8-under-par through 36 holes. Mike Smith (65), Allen Doyle (65) and overnight leader Doug Johnson (72) share third place at 6-under.
Tom Kite is in a group three back following a 5-under-par 67, which included one eagle, eight birdies and five bogeys.
This is the first year that the Hayfields Country Club has hosted this event. The first three tournaments were contested at Hobbits Glen. Fleisher's, Smith's and Doyles 65s are a new course record.
Fleisher, this year's U.S. Senior Open champion, opened his round with a birdie and an eagle, and went on to add four more birdies to his card.
'I have to go at it as I did yesterday and today,' said Fleisher. 'If I go out there to shoot pars, I'm not going to win this golf tournament.'
Fleisher is in search of his fourth win of the season, while Doyle is looking for his second in three weeks.
Thorpe, Smith and Johnson are all searching for win No. 1 this year. Smith and Johnson have yet to win on tour. Smith has a conditional exemption after finishing ninth in the 2000 Senior Tour Qualifying Tournament. Johnson has spent the entire season trying to Monday qualify for tournaments. This is just his fifth start of the season.
Full-field scores from the State Farm Senior Classic