Phil Mickelson

Author Alan Shipnuck's upcoming Mickelson book was the source of controversial remarks Mickelson made about Saudi financiers backing a rival golf circuit.

Shipnuck has released another excerpt of the book on line saying the gambling losses were uncovered by auditors during an insider trading investigation that Mickelson was involved in 2016.

"Mickelson's love of gambling is fundamental to understanding his style of play as a golfer. It might also explain the Saudi seduction," Shipnuck wrote.

That doesn't mean that either, or both, are confirmed to participate in the June tournament, but have reserved their spots should they choose to play.

Woods played in his first tournament in more than a year last weekend at the Masters, finishing 47th, as he works his way back from serious injuries suffered in a car crash in February last year.

He has committed to playing at The Open at St Andrews in Scotland in July, but not in the PGA Championship or the US Open.

At 50 years, 11 months and 3 days old, Mickelson wiped out the mark of oldest winner of a major that was set by Julius Boros, then 48, in the 1968 PGA Championship. Mickelson will turn 51 on June 16.

Mickelson earned his sixth major title, a list that also includes the 2005 PGA Championship. His most recent victory in a major was the 2013 British Open.

His final score of 6-under 282 marks an improbable result.

Koepka, who missed last year's Masters with a wrist injury, used a combination of jaw-dropping power and laser-like irons to card a six-under-par 66 to sit atop the leaderboard with Bryson DeChambeau.