The deadline came two days after GfK, the only rival to WPP, pulled out of talks having been unable to raise financing.
TNS has put out a statement noting the "low level" of acceptances received by WPP and reiterating its recommendation that its shareholders reject the offer.
In extending the offer until 3pm on September 12, WPP has not improved the terms, despite speculation that a higher offer would be likely to win the TNS board over.
The offer, based on WPP's August 29 closing share price of 537.5p, values each TNS share at 274.5p and the whole company's share capital at £1.203bn. TNS shareholders would receive 173p in cash and 0.1889 of new WPP shares for each TNS share they hold.
This is 61% more than the price of TNS shares on April 28, the last business day before TNS announced it was exploring a merger with GfK.
TNS' share price stood at 272.5p as at 8.45am this morning (Monday), down 0.55% from its Friday closing price.