Rory McIlroy and Brian Harman share the lead at rain-hit BMW Championship

Reigning FedEx Cup champion McIlroy, a four-time major winner from Northern Ireland, and US left-hander Harman, who captured the Claret Jug last month at Royal Liverpool, fired five-under par 65s at Olympia Fields in suburban Chicago.

That was good enough for a one-shot lead over five Americans - Sahith Theegala, Chris Kirk, Rickie Fowler, world number one Scottie Scheffler and reigning US Open champion Wyndham Clark - plus England's Matt Fitzpatrick, last year's US Open winner.

McIlroy, who won his third Tour Championship last year, made a tricky chip from the left rough that bounced through a bunker at the par-4 17th and set up his incredible 40-foot birdie chip from behind the green.

"Got lucky. Got through the bunker," he said. "Certainly wasn't expecting to make three when the tee shot was in the air. It was a nice one to make. It was a great bonus."

Only minutes later, Harman holed a birdie chip from 32 feet at 17 to match McIlroy for the lead. That was after sinking a 42-foot birdie putt at the par-3 16th.

"Drove it pretty good," Harman said. "When I missed, I got a little lucky to get it back into play. Chipped in twice, made some putts, played a pretty solid round."

In addition to a $3.6 million top prize, 50 players are chasing a spot in the top 30 in season points to qualify for next week's Tour Championship in Atlanta.

McIlroy, who won last month's Scottish Open, found only three fairways but went bogey-free and made birdie putts from about 13 feet at seven and nine, just inside 10 feet at 10 and, after blasting out of fairway and greenside bunkers, from just beyond four feet at the par-5 15th.

"Luckily, the golf course is soft," McIlroy said. "It wasn't too good off the tee. There were a lot of crosswinds."

McIlroy's new putter, to which he says he's not married, is a good date so far.

"We've been on five dates," McIlroy said. "We're still going OK. We're probably still on for a sixth."

Harman returned last week after enjoying his first major triumph.

"It's nice to get off to a good start," he said. "It was hard to get back into the swing of things but I feel a little more comfortable, a little more ready."

Theegala eagles 14th

Fowler briefly shared the lead but plunked his tee shot into a creek at 17 and made bogey to fall back.

Fitzpatrick was "really pleased" after solving issues with his driver.

"I've struggled the last few months," Fitzpatrick said. "Just feeling more comfortable and that's obviously so important."

Theegala had "one of my best ball-striking rounds" highlighted by a 167-yard eagle hole-out from the 14th fairway with a 9-iron.

"I just had a perfect number," he said. "I knew it was good right away. That was awesome."

Fourth-ranked American Patrick Cantlay, the two-time defending champion, was in a pack on 68 that also included playoff points leader Jon Rahm, the reigning Masters champion from Spain.

American Lucas Glover, the 2009 US Open champion who won PGA titles the past two weeks, opened on 70.

The BMW is the last qualifying event for this year's US Ryder Cup team. Six Americans will seal their spots on points on Sunday for the US squad that will defend the trophy against Europe next month in Italy.