Gary Wilson hit his second LV= County Championship century for Surrey before the hosts suffered a late collapse against Sussex at the Oval.
Ireland international Wilson made 124 from 246 deliveries with 15 boundaries to help Graeme Smith's side reach the close on 301-7.
However their situation seemed set to be even better when they reached 247-3 at one stage of the opening day, only to lose four wickets for 30 runs to the second new ball.
Wilson had been due to bat at seven in the Surrey order but was pushed up to no 3 after Arun Harinath was injured in the pre-match warm-ups.
He made the most of his promotion too, putting on 58 runs with Rory Burns for the second wicket after skipper Smith, who had opted to bat first, gloved seamer James Anyon to fall for just three.
Wilson then combined with Vikram Solanki in a 118-run alliance that eventually came to an end when the latter gloved a leg-side Anyon delivery through to wicketkeeper Ben Brown when on 51.
Wilson was the fourth wicket to fall when he became former Surrey all-rounder Chris Jordan's second victim of the day, sparking a late collapse by the hosts.
Paceman Steve Magoffin trapped Steve Davies (six) leg before and then bowled Jason Roy, who had been summoned back from second XI duty in Derby to take Harinath's place in the team, for a fifth-ball duck.
Anyon gave Sussex a further boost when he had the well-set Zander de Bruyn lbw for 31, giving him figures of 3-69 at the end of an intriguing day of action.