When it comes to knowing the ins and outs of Golf, there doesn't come much better than Butch Harmon.
Putting it simply, what he doesn't know isn't worth knowing.
Along with his father, Harmon was recognised as having the biggest influence on the early part of Tiger Woods' career.
Harmon started working with the golfing prodigy from the age of 17, and in their ten years together, the pair won eight majors along with three U.S Amateur titles.
Along with Woods, Harmon has worked which golfing luminaries such as Greg Norman, Ernie Els, and Phil Mickelson to name just three.
With this level of expertise, the question was recently put to Harmon that who out of Jordan Spieth, Rory McIlroy and Mickelson are most likely to achieve a grand slam of Major wins first.
His response was: "Rory would be the one I would pick out of those three."
McIlroy needs just the Masters to complete the set and came relatively close to doing so last month in Augusta.
Meanwhile, Mickelson requires the US Open and Spieth needs victory at the PGA Championship to join Tiger Woods, Jack Nicklaus, Ben Hogan, Gary Player and Gene Sarazen as the only players to achieve the feat in the Masters era.
With golf quickly moving towards a 'young man's game' it could be deemed at 47, Mickelson's best days are behind him.
He recently won his first tournament in five years (WGC-Mexico Championship), but in-between has battled hernia injuries.
And Harmon feels McIlroy's natural game just gives him the edge over his two American rivals.
When asked if Spieth could potentially find the PGA Championships easier to win than the Masters would be for McIlroy he replied: "Yes, but I think Rory is a better player than Jordan Spieth. I think he has more tools than Jordan, I think that was a momentum killer when he missed that three or four footer on the second hole of Augusta.
"It was his chance to really put the heat on Patrick Reed but it didn’t happen, that’s golf, but I really like Rory’s chances as I think he’s a better player than Spieth."
"Mickelson, I would love to see him win a US Open and complete the Grand Slam, he definitely deserves to be in that category, he’s won five majors, 43 tournaments in the Tiger Woods era which is pretty phenomenal, but Rory is the one who I think will complete a Grand Slam first.”
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