Zarley, who hasn't won since the 1994 Transamerica, offset his double with seven birdies for a 5-under-par round of 67. At 8-under, Zarley leads first-round co-leader Larry Nelson heading into Sunday.
Nelson, who has won twice in 2000, was the only one of the three overnight leaders to post a red number on Saturday. While Nelson carded a 2-under-par 70, fellow 18-hole leaders Simon Hobday and Harold Henning posted rounds of 74 and 77, respectively.
Terry Dill and Jim Ahern are just two shots off the lead at 6-under. Dill matched Zarley's round of 67, while Ahern shot his second straight 69.
The round of the day belonged to Lee Trevino. Vying for his Senior PGA Tour leading 30th career victory, Trevino fired a 7-under-par 65 in the third round at the Nashawtuc Country Club in Concord, Mass. Trevino climbed from a tie for 55th into a tie for 5th. He shares that position with five others.