The world’s biggest ad group also warned shareholders that it could be facing "a changing industry" because of changing client behaviour and new entrants.
WPP said it would review its full-year forecasts in early November but it expects "broadly flat like-for-like revenue and net sales growth" and "headline net sales operating margin improvement" is "now targeted flat".
The group, led by chief executive Sir Martin Sorrell, had said at its half-year results that like-for-like sales growth would be "between zero and 1%" and operating margin would increase by 0.3%.
The UK and Western Continental Europe regions, as well as WPP's advertising and media investment management divisions, were among the best performers.
The group won $2.12bn (£1.6bn) in new billings in the third quarter and $6.36bn in the first nine months, "up significantly" on the first nine months of last year.
Under the heading, "A changing industry?", WPP admitted in today's third-quarter report that all the big ad groups are facing challenges.
"For certain, the advertising and marketing industry has had a good run for the last seven years since the Lehman crisis of September 2008, and a very weak year in 2009, with your company experiencing a V-shaped recovery in 2010 and sequentially record years from 2011 onwards until 2016," the company said in its third-quarter results.
"2017 has, however, been a different kettle of fish, with top line growth slowing across the industry. What may have brought about this significant shift, which seems to have started almost in the first quarter of this year?"
WPP identified three potential factors: advertisers buying directly from tech platforms such as Google and Facebook; the rise of management consultants, which WPP said was over-hyped; and clients cutting costs, such as FMCG giants Procter & Gamble and Unilever.
"It does seem that in the new normal of a low growth, low inflation, limited pricing power world, there is an increasing focus on cost reduction, exacerbated by a management consultant emphasis on cost reduction and the close to zero cost of capital funding of activist investors and zero-based budgeters," WPP said.
Rival groups reported have disappointing growth in the third quarter earlier this month, sending shares in all the big ad groups sliding.
WPP’s revenues fell 2%. WPP prefers to use net sales as a measure of growth because it says revenue includes "pass-through" costs.
Paul Richards, an analyst at Numis Securities, said: "After soft Q3 results from its peers, we believe the market was braced for a challenging quarter for WPP and we expect there will be some relief on revenue trends. Offsetting this, we believe the margin guidance will be taken as disappointing."