Mediate made a hole-in-one on the par-3 third hole en route to a 7-under 64 at the CordeValle Golf Club.
Although Mediate has won five times on the PGA Tour, the last victory coming in 2002, he is best remembered for a thrilling 18-hole U.S. Open playoff with Tiger Woods at Torrey Pines in 2008. Woods won the playoff in extra holes to capture his third U.S. Open title and the most recent of his 14 major crowns.
Ryuji Imada, Bo Van Pelt and Paul Goydos were tied for second at 65 while those at 66 included John Mallinger, Michael Letzig, Will MacKenzie and Shaun Micheel. David Duval was among those at 68, tied for 14th place.
This event is the third of five tournaments that make up the tour's Fall Finish. The tourneys provide the last chance for players who are outside the top 125 on the money list to move inside the cutoff point that determines who will have full-time exemptions to play on the circuit next year.
Notable deaths of 2014 [PHOTOS]