Undefeated world champion Andre Ward opens up about fulfilling his goal of walking away from the sport of boxing on his own volition. (1:58)

Pound-for-pound king and unified light heavyweight world titleholder Andre Ward said in a statement on his website that he is "leaving" boxing.

"I want to be clear -- I am leaving because my body can no longer put up with the rigors of the sport and therefore my desire to fight is no longer there," the statement said. "If I cannot give my family, my team, and the fans everything that I have, then I should no longer be fighting."

On Thursday, the 33-year-old Ward (32-0, 16 KOs) told First Take more about the thought process behind his decision to retire.

"People see what I do fight night," he said. "They see under the lights, but they don't see the toil, they don't see the grind, they don't see just the pain, the physical pain that you go through, not just in the fights, but to prepare and to get ready for those battles."

He added: "I felt the physicality of the sport -- not just in-the-ring stuff, but the training and the preparation -- start to take its toll on me for the last two or three years, and I bit down and continued to push through. And at this point, it's time, and I know it's time."

"I want to be clear -- I am leaving because my body can no longer put up with the rigors of the sport and therefore my desire to fight is no longer there," Andre Ward said in a statement. John Locher/AP

Last month, Ward told ringside media that his television contract with HBO was up. He also said that, contrary to industry chatter, he remains under contract with promoter Roc Nation Sports, even though rumors have swirled that he was gearing for a jump to Top Rank.

Ward has been with HBO since he knocked out Chad Dawson in 2012, though Ward has had two long layoffs since because of promotional issues.

Ward fought six of his last seven fights for HBO, including both of his pay-per-view fights with Sergey Kovalev -- a controversial unanimous decision win in November to take Kovalev's three 175-pound world title belts, followed by an eighth-round knockout victory in the rematch on June 17.

"Andre Ward ends his boxing career as he only knew how to live it -- as a champion at the top," Peter Nelson, executive vice president of HBO Sports, said in a statement. "To watch Ward was to marvel at constant mastery of craft in the ring, to say nothing of his being the consummate role model outside it. The Hall of Fame will be lucky to have him.

"We wish Andre and his family much success and happiness as he explores new opportunities, including with our own HBO family as one of the expert analysts on our broadcast team.

"It was a privilege for HBO to serve as the television platform for many of his landmark achievements in the sport he loves."

Even if his career ends sooner than anyone expected, Ward is a certain first-ballot Hall of Fame fighter.

Ward won an Olympic medal at the 2004 Athens Games -- the last American man to win Olympic gold in boxing -- and quickly rose up the pro ranks. In 2009, Dan Goossen, the late promoter with whom Ward battled over his contract and lost multiple times, secured him a place in Showtime's Super Six World Boxing Classic, a modified round-robin tournament that included six of the world's elite 168-pound fighters.

Given little chance to win the tournament, Ward ran the table in dominant fashion. In front of a hometown crowd in Oakland, California, Ward easily defeated tournament favorite Mikkel Kessler by 11th-round technical decision to win his first world title. Lopsided victories against Allan Green, Sakio Bika (in a non-tournament bout) and Arthur Abraham followed before he easily outpointed Carl Froch in the December 2011 final to unify titles and win the tournament.

Nine months later, Ward returned for another huge victory. Making the move to HBO, Ward faced Dawson, the light heavyweight champion at the time, who agreed to drop down to super middleweight to challenge Ward for his title. Ward issued a beating and knocked Dawson out in the 10th round.

More promotional battles with Goossen followed that kept Ward out of the ring, but when Goossen died in late 2014, his family and Ward reached a settlement on the contract, allowing Ward to sign with Jay Z's Roc Nation Sports. Ward reeled off three wins in a 12-month period, his most active in years, beating Paul Smith, Sullivan Barrera and Alexander Brand -- all easily -- to set the stage for the long-awaited first pay-per-view showdown with Kovalev.

After the second fight with Kovalev, Ward said his contract with HBO had expired. However, Ward and HBO eventually negotiated a new deal. Sources with knowledge of the discussions told ESPN's Dan Rafael it was a four-fight deal under which some of the bouts would be at cruiserweight as well as heavyweight.

A news conference to announce the deal was being planned to take place on Sept. 9 in Carson, California, where Ward worked as an analyst for that night's HBO card headlined by the rematch between junior bantamweight titlist Srisaket Sor Rungvisai and Roman "Chocolatito" Gonzalez. However, the news conference, which had never been officially scheduled, was canceled.

Ward was having second thoughts about the new contract because he didn't want to commit to that many fights, the sources told Rafael. Two weeks later, Ward retired.

While he won't be inside the ring, Ward plans to continue his career as a broadcaster. He was ringside as an analyst on ESPN for the Aug. 19 junior welterweight unification fight between Terence Crawford and Julius Indongo. Ward also will continue to call fights for HBO.

Kovalev said Ward's retirement will be good for boxing because it will allow "good and strong" fighters to compete for the vacated titles. He also said through his Main Events promoter that he doesn't concern himself with the possibility that Ward's retirement won't stick.

"I don't think about him at all because I'm looking forward [to reaching] my goals," Kovalev said, adding that his last two fights are "history" and "I forgot it already."

ESPN's Dan Rafael and The Associated Press contributed to this report.