US PGA Championship: Danny Willett says it is wide open at Baltusrol
|US PGA Championship|
|Venue: Baltusrol Golf Club, New Jersey Date: 28-31 July|
|Coverage: Listen on BBC Radio 5 live and BBC Radio 5 live sports extra, with daily live text commentary on the BBC Sport website|
Masters champion Danny Willett believes the current depth of talent in golf means "anyone" could win this week's US PGA Championship at Baltusrol.
The last four majors have gone to first-time winners, including Willett's triumph at Augusta National in April.
"Anyone who pitches up with their 'A' game has a good chance of winning," said Englishman Willett, 28.
World number one and defending champion Day, American Jordan Spieth and Northern Irishman Rory McIlroy had been expected by some to establish themselves as 'the big three' in the sport, after winning five of eight majors in 2014 and 2015 between them.
As a result of his maiden major at the US Open, Johnson has now been catapulted into a 'big four' - but Sheffield-born Willett believes this year's results prove there is a wide spectrum of players who can compete with the elite.
Rickie Fowler, Patrick Reed, Branden Grace and Brooks Koepka are four talented players who could very easily keep the run of maiden major winners going at the historic New Jersey venue.
"The winners that you've got this year just go to show that it's been so varied. It shows the strength and depth throughout golf at the moment," world number nine Willett added.
Meanwhile, McIlroy is targeting a win this weekend to prevent him from going two years without a major, having won his fourth title at the 2014 PGA Championship.
The 27-year-old said doing so would turn a "B-" campaign into an "A+" one.
"I'd love to sit here and say I'm going to win a major every year," he said. "I could retire at 40 and be very happy.
"There's no reason to think I can't do that for the foreseeable future. I have to play my best golf, but I definitely think it's attainable."
Sweden's Stenson said his Open victory was the "icing on the cake" of his career but he now intends to "focus on what's ahead".
He added: "If I look at my career, a major championship was pretty much the only thing I had not managed to achieve and now I have that.
"But then at the same time, you can look ahead and then try and win another one."
Phil Mickelson, the man Stenson pipped to the title on a thrilling final day at Royal Troon, said he did not want "to let the effects or disappointment linger".
The 46-year-old American, who won the last time the US PGA was staged at Baltusrol in 2005, said: "I think there's a really big window of opportunity to add to my resume, to continue to compete in big events."
World number three Spieth wants to add to his major triumphs at last year's Masters and US Open and will adopt an aggressive approach as he attempts to do so.
"My goal has changed now to trying to win a career Grand Slam, and this would be a fantastic time to grab a third leg," said the American, who turns 23 on Wednesday.
"I've been getting a bit too frustrated at times but recently I've gotten back to kind of the gunslinger, the way that I grew up playing, which is just step up and hit it.
"I have more confidence in my mid-to-long-iron play than I did last year. Short game has gone down just a bit. I'm working hard on it."