Dustin Johnson looked dominant as ever and nothing could stop him Sunday in The Northern Trust.

Not even the weather.

Johnson played the final two holes in near darkness after a late storm delay and finished with a birdie for an eight-under 63 and an 11-shot victory over Harris English.

It was the 22nd victory of his PGA Tour career, and he never made it look easier. Johnson won his fifth FedEx Cup playoffs event - tied with Rory McIlroy for most - and returned to No. 1 in the world. He finished at 30-under 254.

Staked to a five-shot lead over English going into the final round, Johnson sent a towering seven-iron over the water to a front pin on the par-five second, the ball settling eight feet next to the pin for an eagle. Two holes later, his three-wood was placed perfectly in front of the fourth green for a simple up-and-down for birdie.

It was like that all day. It was like that all week.

He led by five and went out in 30, stretching his lead to seven shots at the turn. On the 500-yard 12th hole, where on Saturday he hit a tight draw with a six-iron to a foot, this time he hit a slight fade with a six-iron to three feet for another birdie.

The only drama was whether he could set two PGA Tour scoring records - 31-under par by Ernie Els at Kapalua in 2003, and the 253 by Justin Thomas at the Sony Open in 2017.

But after that birdie on the 12th put him at 29 under, Johnson settled into four straight pars. He is only the third player to finish 30 under, joining Els and Jordan Spieth, also at Kapalua. So he at least owns the record on the mainland.

It was reminiscent of Friday, when he was 11 under through 11 holes and the only question was whether he could become the first player to shoot 57. He made seven straight pars for a 60.

This closing stretch was different. Johnson cares more about trophies than records, and getting his round finished was all that mattered.

There's just no stopping Johnson when he puts all parts of his game together, particularly the irons. Johnson didn't miss a green in the final round.

Kevin Kisner, who grew up playing junior golf with Johnson in South Carolina, has seen this all before.

"He can absolutely dismantle a golf course when he's on," Kisner said. "I've been watching it for 25 years. I'm pretty accustomed to it. When he's on, I just step to the side and try to add to my bank account."

This week's tournament was the first of three playoff events that culminate with the September 4-7 Tour Championship in Atlanta and the $A21 million prize to the FedExCup champion.

Only the top 70 in the standings after this week go on to the BMW Championship at Olympia Fields outside Chicago, and from there the top 30 move on to Atlanta.

Cameron Smith's 69 ensured he was the leading Australian at 12 under, and was tied 17th.

Adam Scott (67) is also safely through to the second playoff event after sharing 58th spot at six under.

Joining Smith and Scott in Chicago will be Jason Day and Marc Leishman, despite the pair missing the halfway cut in Boston.