Deontay Wilder said Anthony Joshua "wasn't a true champion" as the WBC holder hit out at the British star after he was sensationally dethroned by Andy Ruiz Jr.
Ruiz Jr flipped the heavyweight division on its head after stripping previously undefeated champion Joshua of his WBA, IBF and WBO belts in a huge upset on Saturday.
Making his United States debut amid growing calls for a blockbuster clash with rival Wilder, Joshua (22-1) suffered a shock seventh-round TKO at Madison Square Garden in New York.
And Wilder (41-0-1) did not hold back, the American slamming Joshua via Twitter after the bout.
"He wasn't a true champion. His whole career was consisted of lies, contradictions and gifts," Wilder, who is preparing for a rematch with Luis Ortiz, tweeted.
"Facts and now we know who was running from who!!!! #TilThisDay."
While Wilder was scathing, Tyson Fury was far more positive in his response to Joshua's dramatic defeat.
Fury – who will face Tom Schwarz in Las Vegas on June 15th – has described Joshua as a "mediocre average heavyweight" in the past.
However, former unified WBA, IBF and WBO champion Fury sent a warm message to Joshua after Saturday's fight.
"We have our back and Forth's but @anthonyfjoshua changed his stars through life," Fury wrote.
"Heavyweight boxing, these things happen, rest up, recover, regroup and come again."
Joshua has already confirmed he will exercise his rematch clause against Ruiz Jr, with a bout set to take place in either November or December in the United Kingdom.