Sorrell is expected to take the witness stand at the High Court next week as WPP fights a breach of contract case against Benatti, its former country manager in Italy. The case opened yesterday.
WPP has accused Benatti of not disclosing he had a major holding in Media Club when he helped WPP seal a €22m (£17m) takeover of the agency in 2002, earning a 1% commission.
Benatti has denied any wrongdoing and has claimed WPP is using the court process to get Media Club "for free".
The battle between the two sides has run since Sorrell sacked Benatti in January 2006 and resulted in Sorrell bringing a high-profile libel case early last year.
The trial finished with Sorrell accepting £120,000 in damages from Benatti, without the Italian accepting responsibility for circulating a computer-generated image of the WPP chief executive and WPP Italy chief operating officer Daniela Weber captioned: "the mad dwarf and the nympho schizo".