Jason Day opens up a healthy lead in BMW Championship
Jason Day moved closer to claiming the world number one spot after opening up a five-shot lead after the second round of the BMW Championships in Chicago.
Day's 36-hole total of 124 tied the all-time PGA Tour record and left him well ahead of fellow Americans Daniel Berger and Brendon Todd who are tied for second.
Day came close to becoming only the seventh man in history to card a 59 when he resumed his rain-delayed first round on Friday.
Eventually settling for a 61, Day pushed on to record an eight-under round of 63 for an 18-under total as he seeks his fifth tournament win of the season.
Jordan Spieth is seven shots behind Day after a five-under 66 gave him a half-way total of 131, while Rory McIlroy - whom Day will usurp as number one with victory - is a further two shots behind.
Day quickly overcame the disappointment of missing out on a first-round 59 at Conway Farms when he made a barnstorming start to his second round - birdieing the second and third holes before picking up another shot at the eighth to reach the turn in 32 shots.
If his front nine was impressive, his back nine was even better as he recorded four birdies from the 10th before signing off with an eagle at the par-five 18th.
The only blot on his copy book was a bogey at the 13th and Day was understandably happy with his first 36 holes as he goes in search of a fifth title of the year - and second since his clinical triumph at the US PGA Championship at Whistling Straits in August.
"Overall it's been a good last two days for me so I've just got to keep trying to do it," Day told the PGA Tour website.
"Everything feels good right now. I said earlier that if I could describe it (his play) right now it would be 'free'.
"I'm just going out there and trying to get the next birdie and the next birdie after that. I'm just trying to push forward as much as possible. I feel good about my game.
"I've got two more days left. We're going to have a tough day tomorrow with a northerly wind that's blowing from 10 to 20 miles an hour so we'll have to try to stay patient.
"There's two days left and (I'm) trying to win this one."
World number two Spieth could well be overtaken by Day in the world rankings depending on how one another fares this weekend, but the Texan was nevertheless keen to wax lyrical about his Australian rival.
"I feel like I should be paying to come watch this - it was special," Spieth, who finished runner-up to Day at the US PGA Championship last month, said after carding five birdies, an eagle and two bogeys in his round.
"I was very pleased at the way we fought back - being patient, off to a quiet start, watching Jason just continue to go deeper and deeper under par, missing a couple of short putts.
"The way we finished gave myself just a bit of an outside chance but what he's doing right now on the golf course is something I haven't watched or witnessed in my own group live.
"If he continues this pace then he won't get caught, if he lets up a little but then hopefully we can come in."
McIlroy runs the risk of losing his world number one ranking this weekend but he is not going down without a fight.
The Northern Irishman recorded four birdies and an eagle in his six-under-par 65 round - but he feels he should be fewer than nine shots off the lead.
"It definitely feels like I could have been a lot of shots lower than I was," he said.
"(I) hit the ball well from tee to green - that hasn't been the problem over the last few weeks - it's just been converting the chances I've given myself.
"Being six under for the round after 14 holes, with a few opportunities coming up, I put myself in a great position.
"On 15 I didn't make birdie, hit it to three feet on 16 and missed that, hit it to 10 feet on 17 - I hit a good putt there but it didn't go in - and 18 put myself in a great position off the tee and only made a good five in the end so it could have been a lot better.
"But six under is still a good score - a lot of positives to take from it - but I know I'm going to have to shoot a couple of really low ones over the weekend to have a chance."
Justin Rose, meanwhile, is a further shot back on eight under overall after following up his first-round 70 with a 64 which contained eight birdies and a bogey.
Fellow Englishmen Paul Casey and Ian Poulter are six and four under overall respectively while Scotland's Russell Knox is even par.