Lancashire 233 for 4 (Jennings 85*, Wells 55) trail Hampshire 367 (Dawson 86, Prest 85, Lyon 3-110) by 134 runs
Keaton Jennings put behind his disappointment at missing out on England's Test tour to India by beginning his Vitality County Championship campaign with a half-century.
Opening batter Jennings impressed as part of the England Lions squad acting as support for the Test squad in the sub-continent this winter, but was not considered for the main event, with Ben Duckett and Zak Crawley remaining Ben Stokes' and Brendon McCullum's preferred opening pair.
He totted up 85 with only one dropped catch as a blemish in an otherwise authoritative innings, with his opening partner Luke Wells reaching 55.
Lancashire ended the day on 233 for four, 134 runs behind Hampshire's first innings total on a placid Utilita Bowl pitch.
Having bowled the hosts out for what felt like an about-par 367, Wells and Jennings made hay in good batting conditions.
Mohammad Abbas' battle with Wells was intriguing, with the Pakistan fast bowler sending down 16 challenging dot-balls before the batter could manoeuvre himself off strike.
And from that point, the former Sussex opener slowly put himself on top of the home side's bowling attack to rush towards his first half-century of the season.
He found straight driving particularly profitable as he needed just 65 balls to reach the milestone - the 68th fifty of his career.
Wells fell to end an 87-run partnership, of which he had notched up 55 when he clipped Abbas to Tom Prest at short midwicket - in doing so becoming the fifth batter to 50 but not 100.
Jennings was far less aggressive in his approach but never looked in too much danger as the Kookaburra ball quickly went soft.
His main approach to the lack of pace in the pitch was to bat further and further out of his crease, with Lancashire's general tactic of hitting down the ground in opposition to Hampshire's square domination.
Jennings was dropped at point on 36 by Nick Gubbins, who lost his trousers in the process, before slowly closing in on 57th first-class half-century - which eventually arrived in 122 balls.
Josh Bohannon made 30 out of 38 with Bohannon before chopping James Fuller onto his own stumps, after a big build-up of pressure from the Pavilion End - started by Kyle Abbott and continued by Fuller.
Jennings then teamed up with George Balderson in an 86-run stand which appeared to be never-ending until Balderson recklessly slogged to deep midwicket for 38, before Tom Bruce was brilliantly caught at first slip by Liam Dawson to give Holland two wickets in two balls to turn the momentum.
George Bell narrowly avoided edging the hat-trick ball before surviving to the close with Jennings.
Earlier, Hampshire added 62 runs to their overnight score as they extended their first innings by an hour and a half, with three batting points pocketed.
Dawson had gone to bed on 61 having begun his season in the sort of form that 2023 was remembered for, and continued to tick along with Ian Holland and James Fuller in 38 and 45-run partnerships.
Barring a little scamper to reach 350 in plenty of time - which included Dawson pulling Will Williams for six - there was hardly any deviation from a placid tempo.
Holland fell leg before to Williams before Fuller edged a drive off Wells to a helmeted Jennings at first slip, while Dawson fell for an innings-high 86 by a smart catch behind off Tom Bailey. It meant none of Hampshire's four fifty-makers were able to convert to three figures.
Nathan Lyon completed the innings when Kyle Abbott was caught at long-on by Jennings - the Australian ending with three for 110 from a backbreaking 38.1 overs.
From then on in, Lancashire bedded in and made the most of a pitch and ball that was suited for patient long-form batting.
