Valhalla was cloaked in darkness minutes after McIlroy won his fourth major by a single stroke

US PGA final-round leaderboard -16: R McIlroy (NI) -15: P Mickelson (US) -14: H Stenson (Swe), R Fowler (US) -12: J Furyk (US), R Palmer (US) Selected others: -9: L Westwood (Eng), M Warren (Sco), -8: J Donaldson (Wal), J Rose (Eng), -5: D Willett (Eng)

World number one Rory McIlroy edged a gripping US PGA Championship in near darkness at Valhalla to become the first UK player to win back-to-back majors.

Three players were still in contention at the final hole, with heavy rain having earlier led to a two-hour delay.

A see-saw battle on the back nine witnessed four different players sharing the lead at some stage.

But it was the Northern Irishman who dug deep to follow his Open success with a one-shot victory as he finished 16 under.

Rory McIlroy 'proud' after US PGA win

His three-under 68 on the final day saw off Phil Mickelson, with Rickie Fowler and Henrik Stenson a shot further adrift.

The 25-year-old had earlier seen his overnight one-shot lead wiped out on the front nine and was three shots behind the leaders at one point before taking control after the turn.

McIlroy later credited the "best golf of his life" with producing a summer beyond his "wildest dreams".

However McIlroy would not have finished the day were it not for a fine display of sportsmanship from Mickelson and Fowler. The pair were in the group ahead and allowed him to play up behind them as he tried to beat the fading light following the rain delay, then waited on the side of the 18th green to applaud the new champion.

Flashlights illuminated the night sky as McIlroy showed sheer relief after holing out his final putt, fist-pumping the air and celebrating on the course with his father Gerry before receiving the huge Wanamaker Trophy.

McIlroy's resurgence on the back nine capped an enthralling final day and saw him become the third youngest player of the modern era - after Tiger Woods and Jack Nicklaus - to win four of golf's biggest prizes, while he also becomes the first man to win back-to-back majors since Ireland's Padraig Harrington in 2008.

Heavy downpours threatened to take the championship into a fifth day

An ever-changing leaderboard saw an early five-way tie - between Fowler, Mickelson, McIlroy, Stenson and Bernd Wiesberger - transform into a narrow advantage for 25-year-old Fowler around the turn.

The competition then became a see-saw battle with Fowler, playing partner Mickelson, Stenson and McIlroy all taking a share of the lead over the back half.

But McIlroy rediscovered the mental resilience shown in his recent Open win at Hoylake to emerge as a worthy winner.

Having fallen three shots behind Fowler after six holes, he swung the momentum back in his favour with a majestic eagle three at the 590-yard 10th.

BBC Sport golf correspondent Iain Carter "Of his four majors, this was McIlroy's most impressive. He won the first two by eight strokes and took down a classy field at the Open for his third. In this one he demonstrated an astonishing will to win to come from three strokes back on the inward half. His eagle at the 10th showed him capable of golf beyond the rest of the field - no one else hit that par five in two strokes. It was the most thrilling major in recent years - McIlroy prevailed in near darkness and took a giant step towards golfing greatness."

That moved him back to within a shot of Fowler, and he claimed a share of the lead with another birdie on the 13th. And when Mickelson bogeyed the 16th, McIlroy regained the outright lead for the first time since he walked off the second green.

From there, despite reduced visibility as darkness began to fall at the Kentucky course, it was McIlroy's title to lose.

Rory McIlroy raced up the 18th hole to beat the fading light, sealing victory at 8.45pm local time

With the pre-tournament favourite taking a lead into the final day, albeit a narrow one, he was strongly fancied to win a second US PGA title after his success at Kiawah Island two years ago.

However, there had been signs during Saturday's third round that McIlroy was not in the same imperious form that saw him ease to victory at the Open last month.

A stoic McIlroy insisted the rain delay "did not bother him" but he quickly found himself adrift once play resumed at 19:40 BST.

The deluge brought the early starters off the sodden course, forcing a one hour 50 minute delay as water flooded the greens and fairways.

Stat attack The US PGA has the biggest prize fund - $10m (£6m) - of all four majors. Winner Rory McIlroy pockets $1.8m (£1.1m). Runner-up Phil Mickelson, a five-time major winner, had not earned a top-10 finish in any competition this year. McIlroy's Open and US PGA double plus German Martin Kaymer's US Open win mean it is the first time in the modern era that Europe has claimed three majors in the same season. Rickie Fowler is only the third man to finish in the top five of all four majors in a season in the modern era. The American joins Jack Nicklaus (1971 and 1973) and Tiger Woods (2000, 2005).

Ernie Els suggested in the enforced break that play should have been postponed until Monday, but would have been glad that organisers did not heed his advice after making a blistering start.

A run of four straight birdies helped the South African turn in a five-under-par 30, and he gained another shot on 11 before his slim hopes of a fifth major were finally ended with a bogey on the 17th.

Meanwhile, Scotland's Marc Warren carded a 67 to finish on nine under, alongside England's Lee Westwood and McIlroy's playing partner Wiesberger.

The little-known Austrian surprised many to be a shot off the overnight lead, but could only manage a 74, making him one of only two men in the top 30 to shoot over par on Sunday.

England's Danny Willett was the other, scoring a 72 to finish five under, which was three shots behind Wales' Ryder Cup hopeful Jamie Donaldson.

Play was delayed for one hour and 50 minutes as the Valhalla greens staff cleared pools of water

Eventual winner Rory McIlroy started poorly, dropping shots at three and six

Back-half bogeys for American pair Rickie Fowler and Phil Mickelson - on the 14th and 16th holes respectively - handed the initiative back to McIlroy