According to German news magazine Der Spiegel, the former tennis player bought shares in the Nigerian oil and gas industry in 2013, and at one point considered a single investment of more than $10m (£7.6m).

It also alleges he had extensive links outside the world of tennis that ranged from African leaders to Laxmi Mittal, the Indian steel billionaire, and was seen as some one who business dealings went far deeper than celebrity endorsements and media appearances..

The claims centre on documents Spiegel says it was shown by Football Leaks, a whistleblowing website.

They include what purport to be internal emails from Doyen Global, a sports management agency which represented Mr Becker for a brief period starting in late 2013, which reportedly show that Becker Private Office, a company controlled by Mr Becker, held extensive shares in the Nigerian oil and gas business, Spiegel claims.

Doyen Global did not respond to requests for comment.The magazine claims it has seen details of a single investment of “more than $10m” in Nigerian oil prepared for Mr Becker by Forbes & Manhattan, a Canadian investment bank and a former associate of the tennis player. Forbes & Manhattan did not reply to a request for comment.

It is not clear whether Mr Becker went through with the investment, or whether it was successful.

Mr Becker’s association with Doyen was publicly presented as an opportunity for the company to expand its operations in the world of tennis.

But the purported internal emails leaked to Spiegel note that he had met recently with Mr Mittal and John Mahama, the Ghanaian president at the time, and describe him as “some one who can open doors for us in certain circles”.

During bankruptcy proceedings in London last month, John Briggs, Mr Becker’s laywer, told the court: “He is not a sophisticated individual when it comes to finances.”

Mr Becker was not available for comment.