The former tennis star was declared bankrupt in a London court last month, and speculation has been rife over how he managed to squander a personal fortune estimated at over £100m.

Now Germany’s Spiegel magazine has published claims that Becker’s business dealings went far deeper than celebrity endorsements and media appearances.

The magazine claims he made investments in the Nigerian oil and gas industry, 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 someone who could facilitate business deals.

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 Becker for a brief period starting in late 2013, which reportedly show that Becker Private Office, a company controlled by 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  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 Becker went through with the investment, or whether it was successful.