Fonseca (19-1-1) rocked Davis so hard with a punch in the seventh, the former champion had to grab the ropes to remain standing.
Davis came out swinging with aggressive uppercuts to open the bout, trying to end the fight impressively, but he was locked in a tougher test than he anticipated.
Davis, after working out under Mayweather's guidance and training feverishly in a rubber suit Thursday night, revealed again the difficulty some boxers have in making weight.
He apologized on Twitter after Saturday's bout, posting, "I'm young. I'm growing. I had a chance to make the weight. I knew I couldn't make it and that's that. I will have a belt again.
"I lost the belt, not a fight ... . I will make it up."
That left Davis' stablemate at Mayweather Boxing Club, former super-middleweight champion Badou Jack, to fight for a belt. He succeeded, stopping Nathan Cleverly by fifth-round technical knockout for the World Boxing Association light-heavyweight title.
Jack out-landed Cleverly in punches, 172-82, rocking the Welshman with blows in the fourth round, then unloading a barrage of punches in the second half of the fifth and looking to referee Tony Weeks to stop the abuse.
Weeks stopped the punishment 2:47 into the fifth.
"It's a dream come true," said Jack, who is from Sweden. "You can't leave it in the hands of the judges. You've got to go for the kill."
Jack (22-1-2, 13 KOs) said he'd like to fight World Boxing Council champion Adonis Stevenson next, and criticized him for taking a string of unimpressive opponents.
"Let's get real," Jack said.
Earlier, unbeaten cruiserweight Andrew Tabiti improved to 16-0 with a unanimous-decision triumph over veteran former champion Steve Cunningham by scores of 97-93, 97-93, 100-90.
Replacement opponent Yordenis Ugas (20-3) twice knocked down welterweight Thomas Dulorme and proceeded to an upset victory by decision on scores of 93-92, 93-92, 94-91. The loss was clinched because Dulorme (24-3) had two points deducted for low blows in the sixth and 10th rounds.
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