LAKE FOREST, Ill. » Jason Day already had seven birdies on his card and one eagle, courtesy of a shot he holed from 79 yards out of a fairway bunker. He was walking across a bridge Thursday at the BMW Championship when someone asked him during an exchange of greetings, "How are you doing, Jason?"

Jordan Spieth was a few steps ahead of Day, and he could hardly contain his laughter.

"Really? You’re asking him THAT?" Spieth said.

Day has never been better. He powered his way around Conway Farms and was at 10-under par through 17 holes when thunderstorms halted the opening round. His final shot was a 346-yard tee shot with the wind at his back that settled in light rough and left him 44 yards away from a front pin on No. 9.

Day has to hole that Friday morning for a 59. And he didn’t even know it.

"I thought it was a par 72, so I’m sitting there going, ’10 under, there’s no chance at all I can get it.’ But if it goes in, it goes in," he said. "Right now, I’m just trying to play the best I can. I’m just trying to get off to a good start."

He is playing better than anyone at the moment, a winner in three of his last five tournaments, including his first major. A victory in the third FedEx Cup playoff event would send him to No. 1 in the world for the first time, and not many would argue with that.

Day was four shots ahead of PGA Tour rookie Daniel Berger, who had a 6-under 65. Brendon Todd had a 66. Only 17 players completed the opening round.

Spieth finally got on track. Coming off consecutive missed cuts that eventually cost him the No. 1 ranking, he had a hole-in-one on the par-3 second hole to end a peculiar drought. It was the first time in 72 holes that he was under par during any round in a tournament. And then he chipped in from 80 feet for birdie on the next hole.

Little good that did him.

Walking to the fourth tee, Spieth pointed behind him at Day and said, "He’s still the clinic. I’ve barely got the (honors on the) tee."

Spieth made a hole-in-one and Day poured in a 20-foot birdie putt. Spieth chipped in for birdie on No. 3 and Day matched it with a 5-foot birdie. The play was spectacular.

The 27-year-old Australian was as sharp as he has been all year.

He is 89-under par dating to the British Open. Day is playing so well that he said every round feels like practice.

"No matter what you do, even if you hit a bad shot, it’s going to be all right," Day said. "That’s kind of the way it feels. It’s hard to explain because I’m just out there and I’m not really paying attention to the score and I’m hitting it down the middle and hitting it on the greens and holing putts."

It was the best show of the PGA Tour’s postseason, even for a Thursday that was interrupted by a dark and stormy sky north of Chicago. The group of Day, Spieth and Rickie Fowler — Nos. 1-2-3 in the FedEx Cup — attracted an enormous gallery and the players delivered one great shot after another.

Fowler, coming off his third win of the year at the TPC Boston two weeks ago, must have felt like a third-wheel at the end, though he produced four straight birdies on his front nine. When they made the turn, the hits kept coming.

Day’s shot from 79 yards in a fairway bunker on No. 1 landed beyond the hole and spun back a few feet for eagle to get him to 6 under. On the par-3 second, Spieth’s tee shot just covered the bunker, hopped once in the first cut and rolled into the cup for an ace, the second of his PGA Tour career. Day holed a 20-foot birdie putt.

Both made birdie on No. 3, Spieth with his long chip-in and Day with a wedge to 5 feet. Spieth found his groove with a shot into 4 feet on No. 3 for another birdie.

Day’s power was evident on his final few holes.

He tried to drive the green on the 352-yard 17th hole and landed in the collar, 60 feet away. He nearly holed the chip for eagle. On the 600-yard eighth hole into a strengthening wind, he hit his drive 305 yards and then powered a 3-wood high and into the wind. It landed just short of the green, 30 feet from the hole, and he nearly chipped that one in for eagle.

Day gets one more try for an eagle when he gets back Friday morning, a 44-yard pitch for a shot at 59 to tie the course record Jim Furyk set two years ago.

The way things are going, it’s hard to rule him out.

DIVOTS: Rory McIlroy, back to No. 1 in the world, was at 3 under through 12 holes. … Jim Furyk withdrew after six holes with a wrist injury, leaving it doubtful he can play in the Tour Championship next week, and possibly the Presidents Cup depending on the severity of it.