The exiled Russian businessman and former politician Boris Berezovsky told the High Court that Roman Abramovich, the owner of Chelsea Football Club, had been "like my son" and that he was a "genius".
Mr Berezovsky, 65, spoke from the witness box for a second day, where he was cross-examined by Mr Abramovich's barrister, Jonathan Sumption, QC.
He is suing Mr Abramovich, 44, for more than £3bn, which he claims the latter owes him after forcing him to sell his stake in Sibneft, an enormously profitable oil firm, at a hugely discounted price.
Mr Abramovich "is genius", he said. "At least in one way. If he wants to convince somebody personally he may serve them so well. And you trust him so much that you really believe he is sincere. He is genius.
"He really convinced me for a long time he is like my son... He is genius like that, no doubt."
Mr Sumption reminded Mr Berezovsky that he had told the court on Thursday that Mr Abramovich was "not smart". Mr Berezovsky replied: "It's not so."
Laurence Rabinowitz QC, counsel for Mr Berezovsky, said in written submissions to the judge that Mr Abramovich's explanation for paying the businessman £1.3bn was "incredible and incoherent".
Mr Rabinowitz said Mr Berezovsky left Russia in 2000, after falling out with then-president Vladimir Putin and claimed Mr Abramovich "threatened and intimidated" him into giving up his interest in Sibneft for a "mere 1.3 billion" US dollars – "a fraction of its true worth".
He said the judge would be able to consider Mr Abramovich's case – that "Mr Berezovsky [and a third businessman] were no more than gangsters offering him 'krysha': protection" and "never really businessmen" with whom he had been friendly and in partnership.
"Mr Abramovich provides a range of explanations for his decisions to make this payment," said Mr Rabinowitz. "Mr Abramovich suggests that some on-going fear of Mr Berezovsky caused him to pay out 1.3 billion dollars to which (he) had no entitlement in law or in honour."
He added: "The explanations offered by Mr Abramovich as to why he paid this huge sum of money are incredible and incoherent, contradictory and obviously insufficient."
Mr Rabinowitz told the judge: "Mr Berezovsky's case is that Mr Abramovich has lied about Sibneft and lied about intimidation.
"Mr Abramovich's response is to seek to brand Mr Berezovsky the liar. The court will have to determine where the truth lies."
Mr Berezovsky is expected to give evidence for most of next week. Yesterday he appeared far more comfortable than the day before, when he had often struggled to answer questions and denied claims that were later shown to be in his witness statement.
In 2005 Mr Abramovich sold his stake in Sibneft for £7.4bn, which was the largest transaction in Russian history at the time.
The case is expected to last up to three months. Mr Abramovich will give evidence in November.