Who’s No. 1? This guy. Rory McIlroy poses with the Wanamaker Trophy after winning the PGA Championship for the second time. (Mike Groll/AP)

Rory McIlroy was young, grasping and in a hurry. The 25-year-old Northern Irishman practically ran up the 18th hole at Valhalla toward victory in the PGA Championship, eager to get his hands on the Wanamaker Trophy before the day was over. Up ahead, 44-year-old Phil Mickelson stood in the fading light, in no great rush to move aside for him.

There was a 19-year age difference between McIlroy and Mickelson, but on the final leader board, they were separated by just one stroke, 16 under par to 15 under par. Later, when McIlroy is older and we more fully understand who he is in terms of history’s great players, this tournament will be considered one of his most sharply defining moments because he dueled into the twilight with a five-time major champion in Mickelson, who left absolutely everything on the golf course. How often does a man shoot a 66 in the final round of a major championship — and get beaten? But that’s what happened to Mickelson. He was simply bettered by a young great whose round of 68 was a display of astounding talent joined by barely tapped mental and physical strength. Just how good is he? “Better than everyone else right now,” Mickelson said. “Yeah, he’s good. Really good.”

Mickelson was seeking his sixth major title in the space of 10 years, McIlroy his second in the space of a month after winning the British Open jug in July. At 25 years 3 months old, he is now just the fourth player in the span of 100 years to win four majors before the age of 26 — merely joining Jack Nicklaus, Bobby Jones and Tiger Woods. “I’ve still got a long way to go, but to be in their company at this age is very special,” McIlroy said.

The victory turned on a still-echoing 3-wood by McIlroy that led to an eagle he termed “massive,” a low riser of a shot from the fairway of the 590-yard par-5 10th hole. He struck it just when Mickelson seemed to be wrenching the championship away from him. McIlroy had begun this muddy, rain-delayed round with a three-stroke lead over Mickelson and a two-stroke lead over Rickie Fowler, only to lose it when he played the front nine in 1 over. He was unsteadied by the roars generated from up ahead by the attacking Mickelson and 25-year-old Fowler. Speaking of whom, how often does a man place in the top five of all four major championships in a single year and not win at least one of them? But that’s what happened to Fowler. In two of them now he was beaten by McIlroy. “Best player in the world, hands down,” Fowler said.

Mickelson birdied two of his first three holes, while Fowler birdied the third through fifth holes. McIlroy, watching from the fairways just behind them, felt every concussive shout from the large gallery. In the second fairway, he literally had to back off his ball just at address. “I started very tentatively,” he said. “Sort of just trying to get through the first few holes trying to make pars while everyone else was attacking, so that wasn’t good.” Perhaps never in his short career has McIlroy had the heat put on him by two such formidable competitors. “I was paying attention,” he said. “I was paying attention to what was going on. I didn’t want them getting too far ahead of me.”

McIlroy enjoys his fourth career major title with his father, Gerry. (Brian Spurlock/USA TODAY Sports)

But at the 10th, he reasserted himself and made them feel his presence. McIlroy’s screamer of a 3-wood traveled 288 yards. It bounded up the neck of the fairway and onto the green . . . and rolled to just seven feet from the pin. He actually didn’t hit it quite the way intended; it was a little low and left, he said. But with a sharkish look in his eye, McIlroy glided in the eagle putt. Now the roars rolled in the reverse direction up the fairways.

“The eagle on 10 just changed everything,” McIlroy said.

From then on, the back nine was a race against time. Play had been suspended at 12:53 p.m. under a blanket-thick downpour that flooded the course and delayed play. Valhalla’s clubhouse sits on a peak of rolling hills, and rivulets streamed downward as if the course was a water park amusement ride. There were standing lakes on the fairways, and on the greens, the cups were full of water and overflowing.

By the time the leaders went off at 4:19 pm., there was serious question whether there would be enough light to finish a full round. The time constraint only ratcheted the tension across the back nine as players joined the lead, fell out of it and joined again. On the 12th hole, Mickelson made a magnificent saving par putt of 28 feet, with bared teeth and clenched fist. But at the brutally difficult 508-yard par-4 16th hole, Mickelson drove into the wet rough and couldn’t make the green. He flopped a wedge shot that hit the flagstick, half circled the hole and then spun six feet away. His curling putt came up short. He had gone 21 consecutive holes without a bogey. And it came when he could least afford one.

That left McIlroy in the lead alone, ahead by one. By now it was sunset, and he wanted to end things. On the 17th, he struck a gorgeous 9-iron out of a fairway bunker that fell pin high just 12 feet from the flag. Still grim and dead-eyed, he ran the birdie putt into the hole to move 16 under and never cracked a smile, only gave a small fist. He was up two with one hole to play.

The slow pace meant there was a backup on the 18th tee. The light was fading, and there was a roll of thunder ahead. The 18th was a reachable 542-yard par-5 that doglegged sharply and steeply into a stadium-bowl green. Anything could happen there — it had given up 67 birdies and seven eagles on the day. To speed play, Mickelson and Fowler graciously agreed to let the last group play up with them.

McIlroy stepped up and drove into what seemed like a dark cloud bank in the distance. It flirted with a creek bed on the right side of the fairway but stayed dry, and he played from there to a bunker. McIlroy hurried up the fairway, practically crawling up the backs of the men ahead.

Ahead, near the 18th green, Mickelson had a small wedge to the flagstick and knew he had to hole the shot to have any chance. He hit a beautiful high lob that came with four inches of the hole. All McIlroy had to do was put a wedge on the front of the green and two putt, and it was over.

It was the climax of a magnificent summer for McIlroy, who has shed every last vestige of the baby fat and puppyishness he showed in winning his first major, the 2011 U.S. Open at Congressional. The PGA was his third straight victory in the past month. And of the last nine majors he has entered, he has won three of them. “Amazing. Incredible,” he said. “I’m not sure I’ll ever have another summer like this.”

But he has a lot of summers ahead.

For more by Sally Jenkins, visit washingtonpost.com/jenkins.