New world number one Justin Thomas feels he has underachieved by having just one major title to his name.

Thomas, whose victory in the WGC-FedEx St Jude Invitational on Sunday took him top of the rankings for the second time, feels he should have more to show from the game's four biggest events than the US PGA Championship title he won in 2017.

"I could argue that I could have gotten it sooner or I should have gotten it sooner, but I would definitely say I've underachieved saying I've only had one so far in my career," the 27-year-old American said ahead of this week's US PGA at Harding Park where he has been grouped with Rory McIlroy and Tiger Woods for Thursday's opening round.

"I've felt almost getting the second one has been harder than the first one. I don't want to say anybody can win one, but it's definitely harder to win two than it is one. I mean, that's a very obvious statement.

"To get into that other level and to be getting up into the double digits in majors for a career, you need to get going a little bit or you need to start winning some, and when you're stuck on one, it's pretty hard to get there because you've got to get to two first.

"I mean, it's hard to win golf tournaments. It's really hard to win majors. That's the fact of the matter, no matter if you have won them or haven't, it's still tough."