Mediate shot a 9-under 63 on Friday at Hualalai Golf Course, playing the first seven holes in 6 under with four birdies and a chip-in eagle on the par-5 seventh. He added birdies on Nos. 10, 14 and 18 - holing a 15-footer from the fringe on the closing hole.
''I almost didn't play. My left hip is really bothering me,'' said Mediate, a two-time winner last year in his first season on the 50-and-over tour after winning six times on the PGA Tour. ''The chiropractors here are exceptional. I went out and told myself to try and play. It actually got better as the round went on. This came out of nowhere.''
Mediate is marrying longtime girlfriend Jessica Somers at the Four Seasons Hualalai on Monday, then heading to Torrey Pines - the site of his 2008 U.S. Open playoff loss to Tiger Woods - to play the PGA Tour's Farmers Insurance Open.
Dan Forsman, the 2012 winner, was a stroke back. He made a 30-foot eagle putt on the par-5 14th and had seven birdies and a bogey.
''I was really good with the putter,'' Forsman said. ''Very relaxed.''
Couples played the four par 5s in 6 under, eagling Nos. 4 and 7 and birdieing Nos. 10 and 14. On No. 7, his ball caromed off the lava rocks and ended up rolling across the green and stopping 8 feet from the hole.
''It was obviously a big break, probably turning a 6 into a 3 on that hole,'' Couples said. ''The round was OK, but I was only 1 under par on the other 14 holes. I need to do much better. I played four days in a row before today. For me, that's a lot of work.''
The 41-player field features major champions from the last five years, other tournament winners in the last two seasons and sponsor invitees.
Defending champion John Cook had a 69.
Kenny Perry opened with a 72. He was the tour's player of the year last season, winning three times and taking the Charles Schwab Cup points title.