Phil Mickelson's never been shy about his love of gambling. Just how not shy he's allegedly been will drop some jaws.

In an excerpt of his upcoming book Gambler: Secrets from a Life of Risk from publisher The Fire Pit Collective, Billy Walters, a renowned sports gambler and former friend of Mickelson, writes the golfer has bet an estimated $1 billion over the past three decades.

Walters kept tabs on Mickelson's betting during their gambling partnership from 2010 to 2014 and came up with the following figures:

• "He bet $110,000 to win $100,000 a total of 1,115 times.
• On 858 occasions, he bet $220,000 to win $200,000. (The sum of those 1,973 gross wagers came to more than $311 million.)

• In 2011 alone, he made 3,154 bets—an average of nearly nine per day.
• On one day in 2011 (June 22), he made forty-three bets on major-league baseball games, resulting in $143,500 in losses.
• He made a staggering 7,065 wagers on football, basketball, and baseball."

In the most concerning allegations of the excerpt, Walters wrote Mickelson tried to bet $400,000 on Team USA to win the 2012 Ryder Cup. Walters says he talked Mickelson out of the career-threatening bet attempt, though he wrote it was unclear whether the bet was placed elsewhere.

"Have you lost your f--king mind?" Walters said he told Mickelson. "Don't you remember what happened to Pete Rose? You're seen as a modern-day Arnold Palmer. You'd risk all that for this? I want no part of it.''

Rose was permanently banned from baseball and made ineligible for the Hall of Fame after gambling on the sport during the 1980s. He became a pariah across baseball and remains arguably the face of gambling in sports.

However, Mickelson denied placing a bet on Team USA, according to Bob Harig of Sports Illustrated:

"I never bet on the Ryder Cup," Mickelson said. "While it is well known that I always enjoy a friendly wager on the course, I would never undermine the integrity of the game. I have also been very open about my gambling addiction.

"I have previously conveyed my remorse, took responsibility, have gotten help, have been fully committed to therapy that has positively impacted me, and I feel good about where I am now."

Walters estimated Mickelson has lost around $100 million in sports gambling. A biography published last year estimated around $40 million in losses, and Mickelson has acknowledged being "reckless" with gambling in the past.

"My gambling got to a point of being reckless and embarrassing," Mickelson told Sports Illustrated's Harig. "I had to address it. And I've been addressing it for a number of years. And for hundreds of hours of therapy. I feel good where I'm at there. My family and I are and have been financially secure for some time.

"Gambling has been part of my life ever since I can remember. But about a decade ago is when I would say it became reckless. It's embarrassing. I don't like that people know. The fact is I've been dealing with it for some time."

It's unclear if Mickelson still gambles or if he has ever wagered on a tournament where he's been a participant.

Sports gambling has been under an increased microscope since the Supreme Court allowed states to legalize sports gambling in 2018. Several athletes and coaches across multiple sports have faced punishment for betting on their own sport in recent years.