Anthony Joshua has criticised Tyson Fury for calling him a "poor man's Frank Bruno" on social media.
The British heavyweight rivals had a row on Twitter earlier this week, during which IBF world champion Joshua challenged Fury to accept a ringside seat for his fight against Wladimir Klitschko at Wembley Stadium on April 29.
Fury (25-0, 18 KOs) has not fought since a shock points win over Klitschko (64-4, 53 KOs) in November 2015, but that has not stopped his trash talking and abuse of other heavyweights via Twitter.
Joshua (18-0, 18 KOs) was on the receiving end of Fury's latest outburst, during which the Manchester boxer compared Joshua to Britain's former WBC world champion Bruno, who suffered stoppage defeats to the likes of Mike Tyson and Lennox Lewis.
"It's quite disrespectful," Joshua told reporters during a media day at his training base, the English Institute of Sport, in Sheffield.
"If he's an all-time great then why is he worrying about who is behind him? You should focus on your future.
"I just find it so disrespectful. He calls himself the greatest of all time, the GOAT. If I was the GOAT I would never worry about who is behind me. I'm leading the way."
Joshua, the 2012 Olympic gold medallist, says he is willing to fight Fury if he decides to resume his boxing career.
Fury gave up the WBA and WBO belts due to mental health problems -- but Joshua hopes his rival makes a comeback.
"If it were up to me, that would have been the fight [against Fury] happening this summer," Joshua said.
"That's what I thought. I was planning for a massive showdown with Tyson Fury, but he had other plans and I just had to carry on. When he returns, I'm sure I'll still be here.
"Do I want him to come back? Yes, definitely."
When asked who is the biggest fight for him beyond Klitschko, Joshua replied: "I think Tyson Fury, I think Tyson Fury."
But the 27-year-old insists the exchange of insults with Fury has not distracted him from the biggest test of his career against Ukrainian Klitschko, who reigned for nine-and-a-half years until Fury beat him.
"He addresses me and tweets me quite often and I was just in the frame of mind where I thought me and Klitschko don't have much verbals, so let me respond to Fury," Joshua said.