Jason Day has warned critics to write him off at their own peril after an uncharacteristically slow start to the PGA Tour's $US67 million playoffs series.

Day has been a fast starter in previous FedEx Cup finals and even won the first and third events of the 2015 series.

But the former world No.1 Day posted a mediocre result at the Northern Trust event in New Jersey before missing the cut at last week's Dell Technologies Championship in Boston.

However, Day is confident he can repeat the efforts of American Billy Horschel by winning the final two playoffs events to secure the $US10 million FedEx Cup bonus.

Horschel achieved that in 2014, but no Australian has won the overall FedEx title in its 11-year history.

Day starts his quest at this week's BMW Championship at Philadelphia's Aronimink Golf Club.

"If I can do a Billy Horschel and win the last two that would go a long way," Day told AAP on Tuesday.

Day, the 2015 BMW champion, is no stranger to winning back-to-back events on the US Tour.

He first pulled it off during his sensational 2015 campaign before winning the 2016 Arnold Palmer Invitational and WGC-Match Play in successive starts.

"It's definitely realistic; you can catch fire," Day said.

"Sometimes, all it takes is one moment to spark you and ignite that flame and hopefully I get that this week."

While American Bryson DeChambeau has won the first two events of this year's finals, Queenslander Cameron Smith finished third on both occasions to climb to eighth on the FedEx Standings - overtaking Day as the top Australian.

Day, who has won twice on the US Tour this year, has dropped to 10th and is desperate to play his way into the top-five seeds with a good BMW Championship campaign.

The top-five seeds are assured of the FedEx title if they win the Tour Championship finale in Atlanta.

"It is nice to see Cam playing well the last couple weeks, but I'm not too worried about being the top Australian," Day said.

"Bryson has secured the No.1 seed going into the Tour Champs so we are all really playing for the other four spots this week.

"There is a lot of work to be done, and a lot of good players ahead of me and behind me."

Joining Day and Smith in the field are fellow Australians Marc Leishman, the defending champion who won the 2017 edition in wire-to-wire fashion, as well as Adam Scott.

Leishman is 22nd on the FedEx standings while Scott has dropped to 48th.