The notoriously driven adman said was "more important than life and death" to him, and explained his successor would probably not have the same attitude as he has had as the company's founder.
"There have been very few people who have been able to initiate, build and then run a significant company. It is two different sets of characteristics.
"The person who succeeds me is probably more likely to come from within the company than outside. They will never run the business in the same way as me. They may do it better and they probably will do it better, but they will never have the same attitude as me."
Sorrell has no plans to step down, but it is unusual for him to comment on the matter of succession planning, which has been a concern for some WPP investors.
Last year, two independent WPP board directors raised concerns about the lack of a visible succession at the company, as well as the lack of information provided to board members about the aborted bid for rival Aegis Group in partnership with private equity firm Hellman & Friedman.
The succession issue was also raised in 2005 by leading shareholder group RREV, run by the National Association of Pension Funds, and the US corporate governance body ISS.
Sorrell's likeliest successors are viewed as Paul Richardson, WPP finance director, and Shelly Lazarus, Ogilvy & Mather global chairman and chief executive.
Yesterday, the company bought a US digital agency, Schematic, for an undisclosed sum. The agency specialises in media clients including ABC and MTV and was last year engaged by the UK's ITV to redesign and develop its media player. Its revenues for the year ended March 31 2007 were $29.6m (£14.8m).