Floyd Mayweather has warned Amir Khan that he must consent to his drugs-testing demands if they are to meet in the ring.
Mayweather staged an open training session at his Las Vegas gym for British fans who have travelled to the United States to watch Khan fight Zab Judah at the Mandalay Bay Events Centre on Saturday night.
Earlier this week the 34-year-old's uncle and trainer Roger accused Khan of taking steroids, an allegation strenuously denied by the WBA light-welterweight champion.
The British fighter told The Sun earlier this week: "I have been drug-tested so many times I've lost count. And I have never failed one.
"I have been tested out of training camp and in training camp, even answering unexpected knocks at my door to comply. I've proved I'm a clean fighter.
"I am happy to do anything the drug testers want me to do. I have been on Olympic squads as an amateur, so I am very aware of what is involved."
Golden Boy, Khan's promoters, are hoping the rivals will clash late next year.
While refusing to discuss Roger's claim, Mayweather stated that in general any opponent will have to supply blood tests.
"(Roger's accusations) are not my place to say. I don't talk bad about anybody," he said.
"All I'll say is that if you're facing Floyd Mayweather, you have to take the test."
Mayweather has restated that Manny Pacquiao must consent to extra stringent drug testing if they are to meet in what could be the richest fight in boxing.
While non-committal over whether Khan would dispatch one of his former victims, Mayweather was more forthcoming on what must happen if he is to ever fight Pacquiao, who is in the process of suing Mayweather for defamation over previous comments he has made.
The 34-year-old also questioned the progress made by his rival after suffering three defeats and two draws earlier in his career.
"If the tables were turned and I didn't want to take the test, they'd say 'Floyd Mayweather is doing something, no wonder he won all of these fights'," he said.
"All I'm saying is that someone doesn't suddenly become a good fighter at 25.
"Can Floyd Mayweather move to heavyweight and suddenly compete with the Klitschko brothers? Of course not.
"When guys are young you see they have potential, but this guy pops up at the age of 25 and you said, 'Damn, where did he come from?'
"All I'll say is that if you face Floyd Mayweather, you have to take the test."
Inactivity - he has fought just twice in three and a half years with Victor Ortiz next up in September - has seen Mayweather lose his status as pound for pound king to Pacquiao.
Though initially stating he is uninterested in boxing's most prestigious accolade, he soon rails against being dethroned despite having won all 41 of his fights.
"Why aren't I the best when nobody's defeated me yet? You've got this guy that everyone's saying is the best, but he's already been beaten," he said.
"They say I'm too big for Juan Manuel Marquez, but I'm not too big for Pacquiao, yet Pacquiao and Marquez are the same size. That's something I don't understand.
"I can retire without fighting Manny Pacquiao. Floyd Mayweather is not in boxing for Manny Pacquiao, he's in this sport to continue giving his fans pleasing performances."
One such display could come against Khan himself with the WBA light-welterweight champion being lined up to fight Mayweather in autumn next year.
Mayweather, who revealed Khan trained under his uncle Roger at his Las Vegas gym on a few occasions, was not always fulsome in his praise of the 24-year-old, stating "there's nothing there we haven't seen before in boxing".
But he spoke warmly of Ricky Hatton - "a true warrior, boxing will miss him" - and voiced his desire to fight in the UK where he has a large fanbase.
"To still be at the top of boxing after dominating the sport for 15 or 16 years, I'm truly blessed," he said.
"I'd loved to go over and put on a great show and a great fight for my number one fans.
"When I went over there 5,000 fans watched me train. It was truly marvellous.
"I'm like to fight at Wembley, so I'll keep my fingers crossed and hope for the best."
When attention turned to Khan's clash with Judah, Mayweather stressed the American veteran's greater experience could prove crucial.
Mayweather will not be at ringside in the early hours of Sunday morning, instead seeing his family - "I want to spend as much time with my children as possible - I want to be a great champion in the ring but also a great father", but predicts a close encounter.
"I've been in with Zab Judah. Both are explosive fighters. It's a good fight to watch and won't be easy for either man," he said.
"Experience plays a major part in a fight of this magnitude. Zab has been in there with damn near everybody. I don't know who will win."