The tennis star Boris Becker last week retweeted, without comment, a quote from the 2006 film Rocky Balboa. “It ain’t about how hard you hit,” Sylvester Stallone’s Rocky tells his son, “it’s about how hard you can get hit and keep moving forward.”

It’s been a bruising, couldn’t-make-it-up time for Becker, now 50. In June last year, a London court declared him bankrupt, because of his failure to pay a debt, understood to be about £3.3m, owed to the private banking firm Arbuthnot Latham. Becker was and remains furious, insisting that he has not been given enough time to sell some assets to repay the bank. The case was “a gravy train for the suits”, he said. “I will be coming after the people who forced this process through.”

Then, this month, in a head-scratching twist, Becker claimed diplomatic immunity from his debts. If there was widespread surprise in 2013 when he was named as Novak Djokovic’s new coach, this was nothing compared to the shock that accompanied his new gig: Becker had been made the Central African Republic’s attaché for sports, culture and humanitarian affairs to the European Union, the high court of justice in London was told.

Becker, who has homes in Switzerland and London to name but two, has never been to the CAR, which is among the poorest nations, and would not be paid for his role. But, under article 31 of the 1961 Vienna convention on diplomatic relations, Becker’s lawyer argued that he could not be pursued for further payments. The British government would have to request that his immunity was lifted and only the CAR president, Faustin-Archange Touadéra, could grant it.

Of course, the saga didn’t end there. Last week, officials from the CAR claimed they had no record of Becker’s new job. A photograph of his passport was circulated but this showed his position as “financial charge de mission” and not as a sports and cultural attaché.

Moreover, the passport was a “clumsy fake”, according to Charles Armel Doubane, the CAR’s foreign minister. The serial numbers indicated it came from a batch of stolen passports. Doubane’s confusion seemed pretty damning: for any diplomatic passport to be valid it needs to be stamped and signed by him.

And the turmoil continued. Last Wednesday, in central London, a viewing was held for an auction of some of his most prized possessions. The sale, a timed online auction that reaches its climax at 1pm on Thursday, is not at Becker’s instigation; all proceeds go to his creditors.

His whole playing career, which exploded when he won Wimbledon in 1985, unseeded and aged 17, can be traced through the 81 lots. Back then, Wimbledon champions were given miniature trophies to keep and there are three of these in the sale. There’s also a scale replica of the Davis Cup he won with the West German team in 1988 and a full-size replica of the US Open trophy made by Tiffany and Co that he lifted in 1989, after beating Ivan Lendl in the final. This item has the highest bid of £15,100.

Terry Madden, director of Wyles Hardy, which is overseeing the sale, expects some lots to go for significantly more, perhaps even six figures. “You always look at, do they have comparables in the marketplace?” he says. “And in doing my research, I haven’t found any. And the provenance is proven, so I really wouldn’t like to put a final figure on it. I’m hoping I’m pleasantly surprised.”

When Madden met Becker, did he have the sense that he was finding it hard to part with the effects, which also include watches, a pair of tennis shoes and a signed photograph from the president of Germany after Becker won Olympic gold? “Emotionally?” he replies. “Yes, I would have thought it was a fairly emotional situation for him.”

During his tennis career, Becker won six grand slams, including three Wimbledon titles. Photograph: Chris Cole/Getty Images

But then, in some ways, it has been a turbulent couple of decades for Becker. He retired from tennis in 1999, at 31, having won six grand-slam titles and earned about £19m in prize money. Even without adjustment for inflation, he still features in the all-time top 10 for money made playing the sport.

In “retirement”, he has also had success, most prominently as an eccentric commentator and pundit for the BBC’s tennis coverage, which the channel has confirmed he will be doing again this year. His three-year stint as Djokovic’s coach saw the Serb play the best tennis of his career and win six major titles. Last summer, he was made head of men’s tennis for the German tennis federation.

It is his off-court travails and high-rolling extravagance, though, which has made him a figure of tabloid intrigue. In May, in Bild newspaper, it was announced he was separating from his wife Lilly, the end of a 13-year relationship. The split is said to be amicable – “he is a loving father and husband with whom it is never boring,” Lilly said recently – but it came after a sustained period of financial disarray.

Last year, a former business partner, Hans-Dieter Cleven, claimed that Becker owed him more than £30m. That case was rejected by a Swiss court. His divorce from his first wife, Barbara Feltus, is estimated to have cost him more than £15m, as well as their home in Miami. Becker also had to pay £2.4m after he fathered a daughter, Anna, who is now 18, following a notorious tryst with a waitress, Angela Ermakova, on the staircase in the London outpost of the restaurant Nobu in 1999. Though, to clarify: “There was no broom cupboard,” he told Piers Morgan on ITV’s Lifestories in 2009.

There is a sense of sad desperation about Becker’s attempt to claim diplomatic immunity. Not least because the CAR, which has recently endured a civil war and famine, is such a mess. By some measures, it is the world’s unhealthiest country, as well as one of the worst places to be a child.

There is also substantial legal doubt that he would be protected by his diplomatic status anyway. The first mention of any connection between Becker and the CAR came in April; this means, therefore, he is looking to secure retroactive protection, which there is no guarantee he will be given. And why would the CAR risk any kind of diplomatic incident with the UK just to protect Becker?

Right now, Becker’s public pronouncements, made through social media, mainly relate to his concern that Germany might not make it past the group stages of the World Cup. These may, however, soon be the least of his worries. Perhaps Becker can take solace from Rocky Balboa again. “[Life is about] how much you can take and keep moving forward,” Stallone goes on to say. “That’s how winning is done!”