May 23 (UPI) -- Phil Mickelson became the oldest golfer to win a major tournament after capturing the Wanamaker Trophy at the 103rd PGA Championship on Sunday at the Ocean Course in Kiawah Island, S.C.
The 50-year-old Mickelson, who hadn't won a major since the 2013 British Open, passed Julius Boros as the oldest major champion in history. Boros was 48 when he won the 1968 PGA Championship.
"This is just an incredible feeling because I just believed it was possible, but everything was saying it wasn't," said Mickelson, who won his sixth major and second PGA title.
"I hope that others find that inspiration. It might take a little extra work ... but gosh, is it worth it in the end. I'm so appreciative to be holding this Wanamaker Trophy."
Mickelson became the first player in Tour history to win tournaments 30 years apart, with his first victory coming in 1991 when he still was at Arizona State University.
The 2005 PGA champion also became the 10th player ever to win majors in three decades, an honor most recently achieved by Tiger Woods.
Mickelson closed with a 1-over 73, building a five-shot lead on the back nine while avoiding any crucial mistakes to seal the historic win in front of thousands of fans.
The gallery followed him down the fairway -- typically seen only at the British Open -- and erupted after he sank a par putt on the 18th green.
"I don't think I've ever had an experience like that, so thank you for that," Mickelson said during the trophy ceremony. "Slightly unnerving, but exceptionally awesome."
Koepka, a two-time PGA champion, and Oosthuizen couldn't catch up and tied for second at 4-under 284. Koepka closed with a 74, while Oosthuizen hit into the water as he was attempting to make a final run and shot 73.
"Phil played great," Koepka said. "It's pretty cool to see, but a bit disappointed in myself."
Collin Morikawa, who was last year's PGA Championship winner, and eight others finished a shot back of that group for eighth.