Jason Day has joined an elite club on his way to becoming the number one player in men's golf.

The Australian has supplanted Rory McIlroy at the top of the world rankings by winning the penultimate PGA Tour playoff event near Chicago.

Day's also become just the fifth player to win four events in six tour starts.

"The mentality ever since the open championship something changed in my head and I felt like it was my time to start winning tournaments and ever since then it's been a phenomenal run."

He said he's been incredibly nervous.

"Yesterday and today was the toughest round I've ever had to play in my entire life. Just knowing that I had the opportunity to get to number one."

Jason Day has won five PGA Tour events in total this year and holds a commanding lead over Jordan Spieth heading into this week's Tour Championship in Atlanta.

New Zealand number one Danny Lee has dropped to 19th in the tour standings.