(Reuters) - Former All Black Sonny Bill Williams's opponent Francois Botha failed a dope test before their heavyweight bout, local media reported on Sunday, adding further doubts to the credibility of the controversial fight.

Botha, who was beaten on points by Williams on Friday, had tested positive for phentermine, a banned appetite suppressant, the Sydney Morning Herald said in a report.

Williams (6-0) took his unbeaten record to six wins in farcical circumstances when he was awarded the victory over the 44-year-old South African in a bout reduced to 10 rounds from the original 12 at the last minute.

Doubts also surfaced over the fight's status as a purported World Boxing Association (WBA) international heavyweight title bout after local media reported that no officials from the organisation were present.

An incensed Botha, who was well on top in the closing rounds, described the late change of bout length as "match-fixing" and demanded a rematch with Williams, who later said he was more than willing to fight the South African again.

"After, God willing, having a good footy (rugby league) season I'll gladly rematch Mr Botha. Even in South Africa," he said on his Twitter feed.

"Also very humbled by the congratulatory phone call from the WBA on the fight and the win.. To all my supporters nothing but love!!"

(Reporting by Sudipto Ganguly in Mumbai; Editing by John O'Brien)

• Please wait.

• Fernley says costs threaten some F1 teams
• Rosberg tops crash-filled final Monaco practice
• Djokovic and cold weather threaten Nadal's Paris bid
• Rosberg puts Mercedes on pole in Monaco
• Sunshine comes Just-in time as Rose blooms with 69
• Rosberg on pole for Monaco Grand Prix
• South Korea in seventh heaven
• F1 is a superego trip for Wurz's wife
• Make betting legal, says top Indian body
• NBA: Pacers edge Heat to even series