I'm sitting on top of the world - Jason Day

Jason Day expressed satisfaction at proving his critics wrong having been lambasted for targeting the world number one ranking as a teenager.

Day reached the peak of world golf on Sunday with a six-shot victory at the BMW Championship, overhauling Rory McIlroy and strengthening his bid for the FedEx Cup title.

The 27-year-old was ridiculed in 2007 when he revealed his goal of becoming world number one just as he was breaking onto the PGA Tour.

"It wasn't the response that I was expecting. I mean, I expected to get a little bit, but not the response that I got from practically everyone," Day said after his win at Lake Forest, Illinois.

"But it's good to sit in this chair right now, knowing that what I said back when I was 18 years old, it's taken some time, but nine years later, 10 years later, it's worked out. Now I'm sitting on top of the world right now."

Day has won four of his past six tournaments, including the US PGA Championship and The Barclays - the opening event of the FedEx Cup play-offs.

"The way I feel about my game, confidence level has been the highest I've ever had in my entire life, especially as a professional," he said.

"But it's just - it's been an amazing kind of run for me, and now it just tops it off even better just being able to get to number one in the world."

Day shot a two-under 69 on Sunday to win the BMW Championship at 22 under overall from Daniel Berger (16 under) and Scott Piercy (15 under).

The Australian - the youngest player from his country to claim golf's number one ranking - conceded it had been a tough weekend at the Conway Farms Golf Club.

"This week has been a bit of a whirlwind, especially with how I started the week, shooting 18-under par through the first two rounds and then on top of it, the last two days were very, very emotional for me, very hard to sleep at night, knowing that I had the chance to get to number one," he said.