Deontay Wilder believes Tyson Fury “put something hard” in his gloves before their February fight and wants the Briton to “be a man” and fight again.
Fury, 32, beat Wilder in Las Vegas to win the WBC world heavyweight title, 14 months on from them fighting to a draw.
Wilder now says he thinks the Briton tampered with his gloves, a claim Fury already denied in the summer.
“It’s the reason why the side of my face swelled up… and it left a dent in my face as well,” Wilder said.
The American, 35, released a video which included pictures of Ricky Hatton – who was in Fury’s corner – touching his gloves between rounds during their drawn fight in Los Angeles in December 2018.
He then said he feels Fury had a form of weight in his gloves when the Briton stopped him in seven rounds in Las Vegas in February.
“I highly believe you put something hard in your glove,” Wilder said during a video that also contained still images of Fury’s gloves.
There is no evidence Fury has done anything wrong and in July he rubbished questions over his gloves, releasing a video of his hands being wrapped on fight night.
Wilder’s video was accompanied by a succession of tweets in which he called on Fury to honour their “agreement” to fight for a third time.
A trilogy fight was due to take place during the summer but delays have led Fury to seek other plans and state their rematch clause has lapsed.
Wilder tweeted Fury was a “coward”, adding: “Scared people run but a scary man will break his contract.
“It is time for you to be a man and honour your word instead of trying to weasel out of our agreement.”
Wilder has remained tight-lipped since Fury outlined his intention to fight in the UK on 5 December against an opponent who is yet to be named.
Fury has been linked with fighting Carlos Takam – who lost to Anthony Joshua in 2017 and Derek Chisora in 2018.
Wilder questioned the choice of Takam. He said Fury should also consider how he was given his shot at the WBC title in 2018 just months after returning to the ring after time away battling depression.
“When you were going through your darkest time, I told you that if you got yourself together I would give you a title shot,” Wilder tweeted.
“Being a man of my word, I gave you the title shot. When that fight was a draw, I told you that I would give you a rematch.
“You know I was offered more money to fight Joshua than I was getting to fight you.
“Again being a man of my word, I fought you like I said I would.”
Fury and Joshua have agreed a basic financial split for a potential bout between them in 2021, in which all four of the major world heavyweight titles could be contested for the first time in history.
Joshua will risk his IBF, WBA and WBO belts on 5 December when he faces Bulgaria’s Kubrat Pulev.