Spieth’s dramatic Open victory at Royal Birkdale means he needs to win the final major of the year to become the youngest player – and only the sixth ever – to complete a career grand slam.
And although McIlroy has been installed as the bookmakers’ favourite on the back of his improved form and superb record at Quail Hollow, the world number four knows Spieth will be tough to beat.
“He has got that knack,” McIlroy said. “I call it resilience. I don’t know if there’s a better word to describe what it is that he has. But he has got this resilience where he gets himself in positions in tournaments where you don’t think he can come back from, and he does. It’s awfully impressive.”
Spieth took a three-shot lead into the final round at Birkdale but found himself one behind Matt Kuchar after a fifth bogey of the day on the 13th, which followed a wild drive, penalty drop and 20-minute delay before playing his third shot from the practice range.
The 146th Open Championship – in pictures
However, the 24-year-old almost holed his tee shot on the par-three 14th as he played the next four holes in five under par to finish three ahead of a shellshocked Kuchar.
McIlroy added: “You can stand and hit the shots that he was hitting for those last five holes at the Open on the range, no problem. But being able to do it under those circumstances, under the pressure, that’s what makes him so good.
“Resilience, mentally tough, strong, whatever you want to call it. That’s his biggest asset. Being able to forget about a bad shot and move on to the next one, that’s his greatest weapon.”
Spieth goes into the US PGA Championship having won the last major at The Open (Getty)
McIlroy won his fourth major title at the 2014 US PGA but has failed to genuinely contend for another since, despite a share of fourth place in the Open being his sixth top-10 finish.
But the 28-year-old could hardly hope for a better venue than Quail Hollow to end his barren streak, having won his first PGA Tour title here in 2010 and cruising to a seven-shot victory in 2015, which featured a course record of 61 in the third round.
McIlroy goes into the US PGA Championship as favourite despite not winning a major since 2014 (Getty)
“Obviously I haven’t won this year and there’s been a few…