Britain's George Groves will have to rebuild again after losing a third world title challenge, this time against WBC super-middleweight champion Badou Jack in Las Vegas.
Groves was floored in the first round but recovered well, making the fight about even at the halfway stage.
But Sweden's Jack landed the more solid blows in the latter stages of the bout and was awarded a split decision.
Groves was knocked out twice by fellow Briton Carl Froch, in 2013 and 2014.
Groves, 27, stormed from the ring after the verdict was announced but while the fight was close, he was always unlikely to sneak a decision against one of Floyd Mayweather's stable-mates in Las Vegas.
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In addition, while the official punch statistics revealed that Groves threw 215 more punches than Jack, Jack landed with 56 more.
"I thought I won the fight decisively," said Groves. "I thought I controlled the fight with my jab and that I was in control throughout."
Groves, who has lost three of his last five fights, was controversially stopped in the ninth round of his first fight against Froch before being knocked out in front of 80,000 fans at Wembley last summer.
At the MGM Grand, on the undercard of Mayweather's supposed final fight against American Andre Berto, the scorecards were as unsympathetic, reading 116-111, 115-112 and 113-114 in Jack's favour.
The Londoner's latest defeat scuppered a proposed unification match against arch rival James DeGale, the IBF champion, at Wembley Stadium next summer.
The 31-year-old Jack, who was making the first defence of the title he won from American Anthony Dirrell in April, took his record to 20 wins, one defeat and one draw as a professional.
Groves started in positive fashion and appeared to be winning the opening round until an overhand right staggered him and a second sent him to the canvas.
Having popped straight back up, the Englishman received a mandatory eight count while on unsteady legs but managed to make it to the bell.
Groves looked to have made a full recovery by the start of the second, a round he probably shaded, but Jack did the better work in the third, his jab particularly effective.
Groves probably edged the fourth courtesy of his jab but was still unable to establish his right hand, as the champion continued to look unruffled.
The challenger did land with some right hands in the fifth and Groves also outworked Jack in the sixth, making the fight extremely close at the halfway stage.
Perhaps mindful of Groves's reputation for tiring down the stretch, Jack upped his work-rate in the seventh. And by the end of the eighth, Jack's seconds were telling their man that Groves was "done".
While their assessment proved to be a miscalculation, Groves looked even more weary in the ninth as Jack continued to soften him up with some hurtful body shots.
Groves looked very weary by the end of the 10th, Jack having winded him on the bell with a right to the solar plexus.
The challenger fought back gamely in the 11th and also had some success in a wild final round, but Jack was never in serious trouble and probably deserved his victory.