LOS ANGELES (AFP) - Mixed martial arts fighter Conor McGregor switched to the boxing ring to make one of the richest professional fights in history possible and now Floyd Mayweather hints he might return the favour.

Mayweather, who retired after knocking out McGregor in that boxing match last August, teased fans on Friday night (Feb 2) by posting a picture on his Twitter page of himself wearing MMA gloves and standing in an Ultimate Fighting Championship cage.

"Billion Dollar Man," Mayweather said in the tweet.

Mayweather and McGregor's cross-combat fight failed to live up to the hype in the ring as veteran Mayweather easily outpunched the inexperienced McGregor, but it did well at the box office.

The contest generated 4.3 million pay-per-view buys in North America, trailing only Mayweather's 2015 world title boxing match with Filipino Manny Pacquiao that generated 4.6 million buys and earned US$600 million (S$791.7 million).

It is the second time this week Mayweather has teased his fans with the idea.

Earlier this week, he posted video on Twitter and Instagram of himself shadow boxing in a cage.

He said, "Come at the king, you best not miss..."

Mayweather's fight with the Irishman was supposed to be his last as it secured his perfect 50-0 record.

But the American has retired several times, only to negotiate "comeback" fights for bigger paydays.