The 24-year-old Japanese player notched five birdies in his front nine, including a run of four in five holes from the fifth to the ninth.
On the back nine, he was successful with two more birdie efforts, while his only dropped shot of the day was at the par four fourth.
Level with Ishikawa on six-under is unheralded Englishman Brian Davis, who had carded a faultless round of 64.
The 41-year-old started his round with an eagle on the par four first and followed it up with two more birdies on the third and fifth holes.
Davis' low-scoring dried up somewhat thereafter and a string of pars followed before he birdied 17 and 18.
Three players are in the clubhouse on five-under, one stroke behind.
They are England's Greg Owen and American duo Andrew Svoboda and Brendan Todd.