Ali, who made an unbeaten 101 in the second innings of their game with Somerset earlier this week, hit a fine 145 — admittedly against an under-strength second-division county side attack — as the tourists finished the first day on 363 for five.
Veterans Younis Khan and Misbah-ul-Haq made half centuries with Sussex thankful that debutant pace bowler Jofra Archer didn’t suffer from nerves as he returned impressive figures of four for 49.
Indeed the 21-year-old West Indian had the Pakistanis in trouble early on with a two-wicket blast that saw them reduced to 49-2.
But a third wicket stand of 125 between Ali and Younis turned the match back in Pakistan’s favour until Younis fell on 59 to a catch by Ajmal Shahzad off the bowling of Danny Briggs.
Misbah quickly replaced his team-mate and kept Pakistan on track, reaching 221 for three at tea, before he became another victim of Archer when he trapped him lbw on 68.
Ali too was to fall before stumps, Archer having him caught by Ben Brown.