SYLVANIA, July 19, 2014 (AFP) - Diaz finished 36 holes at Highland Meadows Golf Club at 11-under-par 131. She is seeking her third career win on the LPGA Tour, but first in 12 years.
Her only previous wins on the Tour were both in 2002 at the Welch's Championship and the Corning Classic.
"For me, it was just a challenge because I haven't been in this position in a very long time," said Diaz who led by four shots after the first round. "For sure I haven't had cameras on me in a long time.
"So I was anxious for them to go away I think in a funny kind of way."
New Zealand teenager Lydia Ko shot her second straight four-under 67 to move into a share of second place with South Africa's Lee-Anne Pace (68) at eight under.
Canada's Rebecca Lee-Bentham (67) and South Korea's Ryu So-Yeon (67) are tied for fourth at seven-under, four shots adrift of Diaz.
Brittany Lang (66) and Kayla Mortellaro (67) shared sixth at six-under.
Diaz has led this event three times (2002, 2003 and 2009) after two rounds but each time she has faded from sight on the weekend.
Diaz opened her round Friday with five straight pars before making a bogey at the sixth.
Diaz parred her next six holes before pouring in birdies at the 13th, 15th and 16th around a bogey at the 14th to reclaim the outright lead at 10-under.
She would then close her round with another birdie at the 18th to claim a three-shot lead heading into the weekend.
Ko, 17, started her round on the back nine and parred her first two holes before converting three birdies in a four-hole stretch from the 12th to jump to seven-under.
"I had two solid rounds. I thought I played pretty solid today and yesterday," Ko said. "Hopefully I can continue that through the weekend."
Pace, of Paarl, Western Cape, made bogey at the 11th and 15th which dropped her back to minus-six. She rebounded with consecutive birdies at the 17th and 18th to finish at minus-eight.
Defending champion Beatriz Recari (68) is among a group of seven players tied for 14th place at four-under 138.
Leading second-round scores on Friday in the LPGA Tour's Marathon Classic (USA unless stated, par-71):
131 - Laura Diaz 62-69
134 - Lee-Anne Pace (RSA) 66-68, Lydia Ko (NZL) 67-67
135 - So Yeon Ryu (KOR) 68-67, Rebecca Lee-Bentham (CAN) 68-67
136 - Brittany Lang 70-66, Kayla Mortellaro 69-67
137 - Cristie Kerr 70-67, Candie Kung (TPE) 70-67, Lindsey Wright (AUS) 67-70, Katherine Kirk (AUS) 69-68, Mariajo Uribe (COL) 67-70, Caroline Hedwall (SWE) 69-68
138 - Brittany Lincicome 69-69, Mo Martin 67-71, Beatriz Recari (ESP) 70-68, Marina Alex 69-69, Kelly Tan (MAS) 68-70, Mirim Lee (KOR) 71-67, Brooke Pancake 69-69