The 47-year-old American, seeking his first victory on the U.S. circuit since 2002, raced to the turn in a sizzling six-under-par 29 before completing a seven-under 64 at CordeValle Golf Club.
Japan’s Ryuji Imada and Americans Bo van Pelt and Paul Goydos also made the most of ideal conditions for low scoring at the PGA Tour’s Fall Series event, opening with matching 65s.
Americans John Mallinger, Michael Letzig, Will MacKenzie and Shaun Micheel were a further stroke back after carding 66s.
Mediate, who was relieved to find form with his putter after struggling for much of this year, kick-started his round by holing out at the third with a six-iron from 190 yards.
“It carried six, seven steps short of the flag and rolled just barely in,” he told reporters after charging to the top of the leaderboard late in the day. “It was cool.”
An overall total of 27 putts gave Mediate much more reason to smile after a dismal display on the greens in recent months.
“I have been striking it really well for a long time, and I have just gotten murdered on the greens,” he said. “And then it bleeds into everything else.
“And all of a sudden, after hitting a couple of hundred (practice) putts a day on the chalk line, it’s changing. I’m putting the time in on the right stuff.
“Better late than never,” added Mediate, who won the most recent of his five PGA Tour titles at the 2002 Greater Greensboro Classic.
Imada, who has been hampered by a rib injury for much of this year, was delighted with his flawless six-birdie round.
The Japanese professional lies 110th in the PGA Tour money list and needs to stay in the top 125 at the end of this season to safeguard his playing privileges for next year.
“I’m not very comfortable,” Imada said of his position in the money list. “The number is going to be a little bit higher than what I’m at right now, I’m assuming.
“So really I’ve got to have a good week here, and hopefully I can have a really enjoyable week next week in Vegas.”
Former world number one David Duval and South African Tim Clark opened with matching 68s while U.S. Ryder Cup player Rickie Fowler returned a 69.
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