Rob Keogh stepped up in an injury crisis to hit a career-best 158 not out as Northamptonshire showed their promotion credentials with a stirring fightback at the Ageas Bowl.
Keogh's previous top score in six first-class matches was a modest 44, but his perfectly-paced innings after Hampshire had dominated the first session on day one made a mockery of that.
He came into the Northants line-up at the last minute because of the absence of David Willey and then Graeme White, who was hurt in the warm-up, and he responded in the right manner in big stands alongside Andrew Hall and Matthew Spriegel.
Northants, Division Two’s second-placed team, knew they were in for a tough examination after Hall had lost the toss under overcast skies in conditions helpful to the Hampshire seamers.
Left-arm pace bowler James Tomlinson duly struck the first blow by having Stephen Peters caught at first slip and then David Balcombe knocked James Middlebrook's off stump out of the ground with the score at 10.
Keogh came to the crease at 24 for three when Alex Wakeley was caught at the wicket off Balcombe and that worsened to 32 for four when David Sales was deceived by some extra bounce from Matt Coles, the pace bowler on loan from Kent.
But as the sun came out, Hampshire lost their way and Keogh and Hall began the repair process in a major partnership of 120 for the fifth wicket in 34 overs, the former quickly overtaking his previous best.
Hampshire, who were without Michael Carberry and Danny Briggs, both on international duty, called up triallist Ruel Brathwaite from Durham and it was the Barbadian-born pace bowler who broke the stand.
Hall, who made 48, pulled a short delivery to deep square-leg where Michael Roberts held a tumbling catch, running forward.
Steven Crook lifted Brathwaite for six to get off the mark but had not added to his total when he chased a wide delivery down the leg side from the same bowler and Michael Bates, restored to the Hampshire wicketkeeping role, held a catch away to his left.
That put Northants, a team in urgent need of bonus points to close the widening gap on leaders Lancashire, on 162 for six.
Keogh was then joined by Spriegel and together they took the initiative away from Hampshire in an unbroken stand of 161, with former Surrey man Spriegel playing his full part.
Keogh reached his century with a single off Neil McKenzie's occasional spin and by the close the visitors' recovery was looking impressive.
They ended the first day at 323 for six from 96 overs with Keogh having faced 280 balls and struck 23 fours. Spriegel, no less punishing once they came together, was 61 not out with six boundaries and Hampshire regretting not being able to make the most of a promising start.