Jason Day needs a birdie on his first hole today to card an amazing 59
Jordan Spieth launched his BMW Championship bid with a hole-in-one but it was Jason Day who carved out a healthy lead before play was halted for the day due to inclement weather in Chicago.
Day shot nine birdies and an eagle to sit 10 under par after 17 holes as he opened up a four-shot lead over Daniel Berger, with world number two Spieth one of four players one shot further back. Day will card a round of 59 if he can birdie his final hole on Friday morning.
Masters and US Open champion Spieth sits just 0.023 points behind Rory McIlroy at the top of the world rankings and did his chances of leapfrogging the Northern Irishman, who was three under after 12 holes, no harm with an ace on the second hole.
But the day belonged to US PGA Championship winner Day, who kicked off his opening round at Conway Farms with back-to-back birdies from the 10th hole and picked up further shots at the 13th, 15th and 18th - after bogeying the 17th - to reach the turn in 35 shots.
He started his back nine in impressive fashion by eagling the first hole and birdieing the next two before picking up further shots at the seventh and eighth before teeing off at the 405-yard ninth before the horn blew as thunderstorms descended.
Day's 346-yard effort puts him just 44 yards from the hole and if he can secure a third successive birdie he will finish on 59.
The 27-year-old Australian, who leads the FedEx Cup standings ahead of Spieth and Rickie Fowler, told the PGA Tour: "I'm walking through the locker room now and guys are asking me if I'm playing the ladies' tees.
"Jordan told me on nine it was a par-71 and I had a chance at 59. I probably should have known that.
"It feels like a practice round. Like even if you hit a bad shot, it's going to be OK. It's hard to explain."
Day currently leads Berger by four shots after the American finished strongly to record a six-under-par round of 65.
Berger was just two under par with three holes to play before a birdie-birdie-eagle sequence propelled him into second place.
Spieth missed successive cuts at The Barclays and Deutsche Bank Open in recent weeks and might have thought his barren run was set to continue when he bogeyed twice either side of a 13th-hole birdie after starting on the back nine.
The world number two bounced back in perfect fashion, however, when he aced the second hole and then followed that impressive feat with successive birdies at the next two holes.
The 22-year-old Texan picked up another shot at the seventh before parring the eighth as the adverse weather disrupted proceedings.
He joins a four-strong group tied for third on five under, which includes Brendon Todd, who managed to complete his round - which featured six birdies and one bogey - before the rainfall.
World number one McIlroy is one of a 10-strong group - including Phil Mickelson, Danny Lee and Zach Johnson - tied for seventh on three under.
The Northern Irishman parred his first four holes after starting on the back nine but birdied three of the next five holes to reach the turn in 33.
He started the front nine by bogeying the par-four first hole but made amends two holes later when he birdied the third before play was halted.
Meanwhile Jim Furyk was forced to withdraw, just the second time he has had to pull out of a tournament in his career, with a wrist injury after just six holes.
Play is scheduled to resume at 7.30am local time on Friday with 52 players yet to complete their first rounds.