American amateur Bobby Jones completed the original grand slam, the so-called 'Impregnable Quadrilateral', in 1930 by winning the British Amateur, the British Open, the US Open and the US Amateur but that came before the Masters was founded.
Willett, as Masters champion, is the only player in this week's field of 156 capable of going on to complete a calendar grand slam of the majors this year.
"You've got to keep breaking it down. You can't look at it as a whole," the 28-year-old from Sheffield said about taking it one challenge at a time.
"It is quite fun because running up to this week, you are the only guy that can do it in the same year. It's just nice that we have got that chance.
"We're going to try to get prepped, and hopefully come Sunday we're somewhere there or thereabouts to give you that little bit of a feeling that, yeah, this (a calendar grand slam) is actually possible."
Since Willett's three-shot victory at the Masters, where he took advantage of a shocking back-nine meltdown by defending champion and runaway leader Spieth, the Englishman's life has been transformed on and off the course.
"Obviously, you get recognised a little bit more and things like that," he said. "You realise how difficult the guys have had it who have been up there in the limelight for a lot of years, how much they've got to take control of their own time.
"It's tricky when you're in the public eye. The last couple of months, I can't remember a practice session, a golf tournament, where I'm not being filmed or there's a microphone nearby. It's tricky to be yourself."
The US Open starts on Friday.
- AAP
