Floyd Mayweather Jr. was thought to be retired from boxing after winning all 49 of his professional bouts, but the allure of $100 million for a fight against Conor McGregor, the two-division titlist in the Ultimate Fighting Championship, proved too tempting to pass up.

However, the 40-year old Mayweather says the August 26 bout against McGregor, 28, at T-Mobile Arena in Las Vegas will be his last.

“I told Al (Haymon) when we had a meeting, I can’t push my body,” Mayweather said at the opening press conference of the four-city promotional tour. “Training camp is grueling. I can’t do it anymore. It’s rough. Training camp is rough. I’m just working, going to the gym, working hard every day. It’s rough. This has to be my last one.

“After taking a couple years off, I was okay, but then, we somehow ended up making this fight happen, and now, I really know that this is it. Deep in my heart, this is it. I can’t.”

In his illustrious career, the 5-foot-8, 151-pound Mayweather won the WBC super featherweight, lightweight, welterweight and light middleweight crowns, along with the IBF welterweight title, WBA (Super) light middleweight and (Super) welterweight titles and WBO welterweight championship.

On his way to the 49-0 record, Mayweather defeated Manny Pacquiao, Canelo Alvarez, Miguel Cotto, “Sugar” Shane Mosley, Juan Manuel Marquez, Marcos Maidana (twice), Arturo Gatti, Ricky Hatton and Oscar De La Hoya, all of whom are current or were former champions across several weight classes.

“I’ve been off, and I said this on social media, I’m not the same fighter I was 21 years ago,” Mayweather said. “I’m not the same fighter I was 10 years ago. I’m not even the same fighter I was two years ago. I understand that, but I have that fighter’s mentality.”

Last November, McGregor came to Madison Square Garden in New York City looking to make history and become the first simultaneous two-division champion in the history of the UFC, and that is exactly what he did.

Courtesy of a knockout of Eddie Alvarez in the main event of UFC 205 at "The World's Most Famous Arena," the 5-foot-9, 155-pound McGregor added the UFC lightweight championship to his resume, going along with the featherweight title he won in December of 2015.

While taking a right-handed punch from Alvarez, McGregor fired off a powerful left hand that landed on the side of the defending champion’s face. McGregor followed it up with a right hand to Alvarez’s jaw, a left to the ear and another right that eventually floored his opponent.

And in order to prepare to take on the young, hungry McGregor, who has never boxed professionally, Mayweather is doing what is necessary to make sure his body is ready to go by the time the boxing match arrives next month.

“Knowing my body in training camp, I’m taking a lot of time off, letting the body rejuvenate, letting the body heal, and just to make sure that before I go out there and compete, whether it’s in that boxing gym or in that squared circle, I’m going to make sure that my body is as close to 100 percent as possible,” Mayweather said.

“I’ve been letting my body do a lot of rest. I’ve been doing a lot of resting this training camp, whereas in other training camps, I really wasn’t resting as much.”