Talking to global business broadcaster CNBC, the just-turned 65-year-old leader said last year had been "a game of two halves" for WPP.
Sorrell said: "In the first half of the year I think we were staring into the abyss. In the second half we certainly saw things becoming less worse."
Sorrell said the company, which put in a hiring freeze last year as part of a cost-cutting exercise, was now hiring again.
Revenue across the group, which includes MediaCom, Mindshare and Grey, fell last year, with the UK reporting a 6% drop compared to the previous year, while revenue in western Europe fell by 10.2%.