World number one McIlroy could lose his world No 1 crown at in the weather-dealyed BMW Championship after Jason Day rocketed four clear of the field with one hole to complete.
To add spice to the Australian’s bid for victory that would give him the world No 1 spot and a fourth win in six starts, he needs to hole his 44-yard second shot from the rough on the ninth at Conway Farms Golf Club in Illinois to card an opening 59.
At 10 under par, Day is four shots clear of American Daniel Berger, who eagled his final hole, the par-four ninth, for a six-under 65 in breezy conditions with playing partner Jordan Spieth in a four-way tie for third with fellow Americans Brendon Todd (66), Kevin Chappell (16 holes), and Justin Thomas (13 holes).
Day, who could leapfrog Rory McIlroy and Spieth to reach the top of the rankings with victory this week, was in ominous form before play was suspended for the day due to the threat of lightning.
Day, who won the Canadian Open, PGA Championship and The Barclays in his last five starts, chalked up four birdies in his first six holes and though he gave a shot back on the 17th with a three-putt bogey, the 27-year-old then caught fire.
He birdied the 18th, holed out for eagle from a fairway bunker at the par-four first and added back-to-back birdies at the second and third to storm three strokes clear.
Day picked up further shots at the seventh and eighth to get to 10 under and raise hopes of a possible 59, then hit his tee shot at the ninth into the right rough before the siren sounded to halt the action under threatening skies.
Spieth was unable to keep pace with the red-hot Australian, despite registering a hole-in-one at the second and a chip-in birdie at the third, and was five under for the round after 17 holes.
World number one McIlroy, back from a break in Bermunda and the Bahamas which included some swing work and a chat with putting coach Dave Stockton, was at three under after 12 holes in an elite field of 70 players from which just 30 will advance to the Tour Championship finale next week in Atlanta.
PGA Tour veteran Jim Furyk, who shot a 59 two years ago at Conway Farms in the second round of the BMW Championship, withdrew from the tournament with a wrist injury after completing just six holes in the opening round.
McIlroy started on the 10th and missed chances at the 11th and 13th before finding his range and showing that he’s not far away from putting his full game together, even if his left ankle is still far from 100 percent.
He holed a 15 footer at the par-five 14th, a 12 footer at the 16th and a 10 footer for another birdie to turn in 33 when he might easliy have picked up another three shots.
A three putt bogey from 20 feet at the first stoped his momentum before he rolled in a six footer for birdie at the third to get back to three under for the day.
He faced a 12 footer for another birdie at the fourth when play was suspended.