Jordan Spieth believes he faces a battle to hold on to his new position as world number one from a "fearless" generation of young players.
Masters and US Open champion Spieth replaced Rory McIlroy at the top of the rankings after finishing runner-up to Jason Day in the US PGA Championship at Whistling Straits on Sunday.
Day's first major title also lifted him to third in the latest standings and means the world's top three players are aged 22, 26 and 27 respectively.
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The 27-year-old Branden Grace is ranked 20th after finishing third at Whistling Straits, with the 25-year-old American Brooks Koepka up to 17th after sharing fifth place on Sunday. Players Championship winner Rickie Fowler, 26, is ranked eighth.
"With so many people being so fearless, they commit to those aggressive lines and then pull off the shots. That's tough," said Spieth, who was surprised by how aggressive playing partner Day was in Sunday's final round.
"With the breeze being up, I didn't think that everybody would be significantly under par. I looked at the board and said, 'Wow'. I was just amazed at how great the golf was this week. This was not an easy golf course.
"For that many people (16) to get to double digits under par means that there's some incredible golf being played and what that does for me at this point is now I realise that although we have just reached that goal of being number one in the world, with the way these guys are playing and the way you see it being played on the biggest stage, it's going to be really, really hard to keep that position.
"That's how I think of it with the game's young players. It doesn't matter apparently if you've won a major or not before. Here we did what we could and it wasn't enough because Jason just played that good."
Day caused a stir in his rookie season in 2007 when he said he could one day "take down" Tiger Woods, but although Woods is now 286th in the rankings, the 27-year-old still wants to become world number one.
"I still want to accomplish that number one goal of mine, which is to be the best player in the world," Day said. "I'm still motivated and still very hungry for that, even after this win.
"I think golf is in a very healthy stage now. Three to five years ago it was kind of struggling a little bit with the identity of who was really going to be that number one player in the world, who was going to be the next best thing, and kind of Rory came out and was really dominating. But there was no-one really challenging him for that role.
"For young guys like myself and Jordan and Rickie Fowler and Hideki Matsuyama, a lot of those guys are starting to play better golf and starting to challenge. So that's what I'm looking forward to in the future, the shear competition of being able to fight against these guys each week and have that competition and fight against them. It's going to be a lot of fun over the next five to 10 years."