The WPP leader said the group had seen sequential monthly improvements since the start of the year, culminating in "a little bit of growth" of 1% in March.
WPP reported revenues of £2.1bn, down 1.82% year on year, during the first quarter of 2010.
In an interview with CNBC he also admitted "sequential growth" is expected to continue in April, May and June.
In the interview, which follows WPP’s trading statement issued today, Sorrell is also questioned on whether WPP will return to Britain if the Conservatives are elected.
Sorrel does not confirm or deny a move. He said: "If it's on the basis of a manifesto we couldn’t make a decision. It's not consultation.
"What we need is a guarantee that overseas profits will not be taxed. That's what drove us to Dublin."