He thought he could win his inaugural start in the Allianz Championship at Broken Sound. But he admitted he had no idea.
''I put the work in, but you don't really know what's going to happen,'' Mediate said. ''I know how good the guys are on the Champions Tour.''
Mediate didn't take long to prove he could play with the 50-and-older set. He shot an 11-under 61 in the second round, the lowest of his 29-year pro career and a tournament record. It gave him a three-shot lead he parlayed into a two-shot victory over Bernhard Langer and Tom Pernice Jr.
Mediate became the 16th player to win his Champions Tour debut.
''It was great, honestly unexpected,'' said Mediate, who won six times on the PGA Tour, but may be best known for his spirited playoff loss to Tiger Woods at the 2008 U.S. Open at Torrey Pines. ''To have it happen the first week was kind of wild, actually.''
Mediate's first PGA Tour victory also came in South Florida at the 1991 Doral-Ryder Open, when he became the first player to win with a long putter he used to alleviate back pain.
On Wednesday, Mediate played a practice round at Doral's renovated Blue Monster course with new owner Donald Trump.
Mediate won win another Champions Tour title and claimed Rookie of the Year honors in 2013 after earning $1.34 million, more than he made in any season since 2008. He's also enjoyed success off the golf course, marrying Jessica Somers last month.
At 51, he insists he's playing the best golf of his career. He'll attempt to become the first player to successfully defend his title since Hale Irwin at the 2005 Turtle Bay Championship in the Champions Tour's first full-field event.
''I think every single part of my game is improving,'' Mediate said. ''I hit it farther than I ever hit it, I think my technique's better, I definitely putt way better than I ever putted, with any putter. I definitely think I'm a better player than I ever was, especially around the greens.''
Mediate isn't the only player trying to make history this week. Langer, who lives 5 miles from Broken Sound, will be trying to become the 10th player in Champions Tour history to reach 20 career wins.
Langer has a good chance this week, having finished among the top three in his hometown event four of the last five years, including holing a plugged bunker shot for an eagle to beat John Cook in a playoff in 2010.