ILLINOIS: Jason Day, in pursuit of world No 1 status and a fourth win in six starts, was poised for a possible 59 after charging four shots clear in the weather-hit opening round of the BMW Championship on Thursday.

Day, who could leapfrog Rory McIlroy and Jordan Spieth to reach the top of the rankings with victory this week, produced sizzling form as he moved to 10-under par after 17 holes before play was suspended for the day due to the threat of lightning. That left him a commanding four strokes ahead of American Daniel Berger, who eagled his final hole for a six-under 65.

Day had just hit his tee shot 346 yards into the right rough at the ninth before the siren sounded to halt the action and he faces a second shot from 44 yards for what would be a sensational eagle. Should he hole out from there when he returns to the course to complete his round, he would become only the seventh player on the PGA Tour to shoot a 59.

Masters and US Open champion Spieth was part of a four-way tie for third at five under as he tried to keep pace with playing partner Day in the third of the PGA Tour's four concluding play-off events in the season-long FedExCup race. Spieth was level with fellow Americans Brendon Todd, who had completed 16 holes, and Justin Thomas, after 13.

Day chalked up four birdies in his first six holes and though he gave a shot back on the 17th with a three-putt bogey, then caught fire. He picked up further shots at the seventh and eighth to get to 10 under and raise hopes of a magical 59, then hit his tee shot at the ninth before play was suspended.

World No 1 McIlroy 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.