Sussex 112 for 2 (Haines 53*) trail Middlesex 271 (Robson 113) by 159 runs
Sam Robson's superb 113 off 103 balls spearheaded Middlesex's sprint to 271 all out from just 44.3 overs at Hove, with Sussex replying on a more sedate 112 for two in a Vitality County Championship match that lost its first seven sessions to bad weather.
Tom Haines' secure unbeaten 53 steered Sussex through 33 overs in the later stages of a day blessed at last by early autumn sunshine, leaving them with the prospect of clinching the Division Two title on the final day of the season.
Fast bowler Tom Helm dismissed both Oli Carter for 15, caught and bowled top-edging an attempted pull, and Tom Clark, who snicked behind on 14, but Tom Alsop (11 not out) stayed with Haines until stumps.
The game finally got under way at 12.40pm on day three, following two inspections and an early lunch, and Robson led from the front as Middlesex tried to make up for lost time in a valiant effort to reach a position from which to push for an unlikely victory.
Middlesex had started the day still with an outside chance of promotion, needing to win in this final championship round while also requiring second-placed Yorkshire to lose against Northamptonshire. But Yorkshire's three bowling and three batting bonus points at Headingley meant they go up alongside Division Two leaders Sussex - who now need just one batting point, or a draw in this game, to become second division champions.
Picking up three bowling bonus points, with young spinners Jack Carson and James Coles sharing seven wickets, left Sussex at stumps just 138 runs away from reaching 250 and that first batting point. Sussex have announced that all spectators will receive free entry on the final day, plus a voucher for a free drink.
Robson's hundred, the 36th of the former England Test opener's fine first-class career, was his fourth of an excellent season and a chanceless innings of controlled aggression. The 35-year-old reached three figures off 87 balls and hit 15 fours.
With Ryan Higgins including three legside sixes in a 28-ball 40, Middlesex were at one stage 181 for three. But Higgins then swept slow left-armer Coles high to deep square leg, in the 29th over, and Middlesex's first innings fell away despite every batsman attempting to maintain the attacking intent.
Coles finished with four for 61 while off spinner Carson's three for 46 took him to 50 championship wickets for the season, the first time a Sussex spinner has reached that landmark since Monty Panesar's haul of 53 in 2012.
Opening bowlers Ollie Robinson and Jaydev Unadkat claimed Middlesex's first three wickets, with Robinson removing Mark Stoneman for 13 - courtesy of a diving low catch to his right at fourth slip by Haines - in his new ball spell of 7-0-21-1.
Indian left-armer Unadkat then replaced Robinson for his second spell from the Cromwell Road End, almost immediately bowling Max Holden off stump for 24 and later having Leus du Plooy (6) held after a juggle at first slip by Alsop.
By then Robson had completed a 51-ball fifty by straight driving Unadkat for four and Higgins' arrival sparked a rollicking fourth wicket partnership of 78 from just 8.3 overs.
Coles, though, followed up his dismissal of Higgins by having Josh de Caires (1) well-held by Carson at deep square leg, who in the previous over had bowled Jack Davies through an attempted big drive for four.
After tea, taken at 208 for six from 36 overs, Robson swept Carson to deep backward square leg before Toby Roland-Jones (8) mishit Coles to mid off, Helm (7) was caught off Carson and Luke Hollman's bright 34-ball 39 was ended when he was bowled swinging at Coles.
