The short-lived union of Jason Day and renowned caddie Steve Williams appears to be over.

Day and Williams joined forces ahead of the U.S. Open at Pebble Beach two months ago, but according to an AAP report the two men parted ways in the wake of Day's missed cut at The Northern Trust following rounds of 72-70.

"Parting with Steve was a mutual decision and not an easy one to make," Day said in the report. "He is the ultimate pro; it was a real learning experience being exposed to a caddie who has achieved what Steve has. He has worked with the best. But it was a disconnect of old school and new school."

Williams was on the bag for 13 of Tiger Woods' 15 major titles, and he also helped Adam Scott to victory at the 2013 Masters. He last caddied full-time for Scott in 2017, but he ended his latest stint of retirement to join forces with another Aussie and former world No. 1.

Day told reporters at Pebble Beach he believed he had "severely underachieved" in his career to date and expressed hope that a long-term relationship with Williams would be part of the solution moving forward.

The Northern Trust: Full-field scores | Full coverage | FedExCup standings

"He said that being No. 1, there's a lot of sacrifice that comes along with that. And we both understand what that means," Day said in June. "So I'm very excited about this week and the potential of many years to come."

But Williams' time with Day was much less fruitful than his six-year stint with Scott, as Day missed the cut in two of six starts with Williams on the bag and recorded just a single top-20 finish (T-8 at the Travelers Championship). After starting the year ranked 14th in the world, Day is now down to 22nd – his lowest ranking since June 2013.

Day has had multiple caddies since splitting with former looper and current swing coach, Col Swatton, nearly two years ago, and he'll reportedly have friend David Lutterus on the bag this week at the BMW Championship in Chicago. It's a pivotal event for Day, who has fallen to ninth place in the International Presidents Cup standings entering the final week of qualification with only the top eight automatically earning spots on Ernie Els' squad at Royal Melbourne later this year.

Day also dropped from 40th to 50th in the FedExCup race following his early exit at Liberty National, meaning he'll need a strong result at Medinah to move into the top 30 and avoid missing the season-ending Tour Championship for the first time since 2012.