According to a report in The Daily Telegraph, Sorrell and Lowden are believed to have discussed potential savings from a TNS takeover, which WPP has so far estimated at about £52m.
While the meeting provides a signal that TNS is relaxing its hostility to WPP's interest, Lowden is also reported to have asked advisers Deutsche Bank to sound out other potential buyers, including Nielsen and US private equity company Hellman & Friedman.
WPP made a £1.08bn hostile bid for TNS on July 9, offering 260.6p per share, payable in cash and WPP shares. The marketing services group's agressive courting of TNS has already brought about the collapse of the merger TNS and market research rival GfK had agreed.
GfK is attempting to find a partner to finance an acquisition of TNS, and it emerged that it has been talking to private equity firm Cinven as well as German billionaire Gunter Herz about making a joint bid.
GfK needs between £400m and £600m from a joint venture partner in order to come up with a better offer than WPP.