Friday, 15 September 2017: With a new caddie on the bag, Jason Day has made a hot start to the BMW Championship in Chicago.

Jason Day has deemed his first hit-out with a new caddie a success after starting the BMW Championship in fine style.

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On tournament eve, world No.9 Day parted indefinitely with long-time caddie Colin Swatton and placed close mate Luke Reardon on the bag for the penultimate event of the $US67 million FedEx Cup play-offs series.

The 29-year-old Day seemingly clicked with Reardon, whom he met at high school in Queensland, firing a seven-under-par 64 to sit in a tie for second and two shots back of leader and fellow Australian Marc Leishman.

"It was a good step in the right direction; seven under is a great start," said Day.

Leishman carded a stunning 62 to climb to nine-under at Conway Farms Golf Club.

Day, who had Swatton on the bag for all 10 PGA Tour wins including the a breakthrough major at the 2015 US PGA Championship, says Reardon performed admirably.

"Not having Colin on the bag was different and I didn't know how I was going to play, but I think Luke did a fantastic job out there," said Day.

"We worked well together and hope we can keep that going."

Day's decision came as a surprise given he and Swatton teamed up to tie for ninth at the US PGA Championship in August, his first top-10 at the majors this year, before a share of sixth at the Northern Trust and a top-25 at TPC Boston two weeks ago.

Ranked 28th in the FedEx Cup standings, Ohio-based Queenslander Day needs a top-30 finish to advance to the 30-man Tour Championship finale in Atlanta next week.

Day says he'll draw from the positive memories of his six-shot victory at the BMW Championship in 2015, also played at Conway Farms that year.

"Being able to have some good experiences here in 2015 is good for the confidence," Day said.

"I won here and I know I can do it again. It's a great golf course and it sets up nicely for me."