Hampshire captain Jimmy Adams did what Northamptonshire's Rob Keogh had done over the previous two days, grafting for a double-century that seems certain to consign the LV= County Championship Division Two match at the Ageas Bowl to a high-scoring draw.
Adams batted for a total of nine and a half hours for 218, his best score of the season and only three runs fewer than Keogh - whose innings had been responsible for Northants' formidable total of 438.
With Adams in control on a bland and lifeless wicket, Hampshire ended the third day 77 ahead at 515 for nine and, with rain forecast, it is hard to see Northants finding a way of winning their sixth championship match of the season to maintain the pressure on leaders Lancashire.
Hampshire began the third day at 160 for two with their only objective being to score another 129 to avoid having to follow on, but that was never in doubt as Adams and Neil McKenzie put on 102 in 32 overs for the third wicket.
Even when McKenzie was leg before to Muhammad Azhar Ullah at 235 for a 10-boundary 52, Adams was unmoved and reached the 19th century of his first-class career, and one of the more patient as Northants wilted in the sun.
Adams lost Sean Ervine at 278, bowled by Steven Crook, and Adam Wheater, his stumps disturbed at 331 by Trent Copeland, but he moved serenely to his double-century by lifting the off-spin of James Middlebrook for a straight six to reach the landmark.
There was no respite for Northants as Michael Bates joined Adams in a partnership of 152 in 41 overs for the sixth wicket as Hampshire went into the lead in the 138th over and began to think in terms of building a commanding advantage.
Bates, getting a rare chance behind the wicket with Wheater playing as a specialist batsman, hit 10 fours in his 71 before cutting Azhar to Copeland at point and all that remained for a small basking crowd was to see if Adams could bat through the entire day.
Middlebrook had Matt Coles, on loan from Kent, caught at first slip by Andrew Hall without scoring at 486 and finally, 10 runs later, Adams prodded wearily at Matthew Spriegel's spin and Hall again held on behind.
Adams faced a total of 444 balls and hit 35 fours in addition to his six and when he departed Hampshire were 58 ahead.
Ruel Brathwaite got off the mark by hitting Spriegel for six but in the last designated over the West Indian gave Middlebrook a return catch.
Northants did not help their cause by conceding 56 extras and gifted Hampshire five penalty runs when wicketkeeper Jonathan Batty, making his debut for the county, threw his glove at the ball and made contact with it.
Azhar, Crook, Middlebrook and Spriegel each took two wickets but Essex's victory over Worcestershire earlier in the day put more pressure on Northants to find a way of winning this, their 14th of 16 fixtures.