Middlesex 138 and 9 for 0 trail Sussex 481 for 9 dec (Van Zyl 173, Brown 107, Salt 50, Beer 50) by 334 runs
Stiaan van Zyl's marathon hundred set Sussex on course for a maximum-points win over Middlesex on day two at Lord's.
The South African left-hander batted for just over seven and a half hours for his 173 and with wicketkeeper Ben Brown weighing in with 107, Sussex, who bowled out the hosts for 138 on the opening day, piled up 481 for 9.
Middlesex, for whom James Harris returned figures of 4 for 98, survived four overs before stumps reaching 9 for 0, but they face the prospect of having to bat out almost all of the remaining two days to salvage a draw.
Sussex were only 31 ahead at start of play, but van Zyl and Brown showed great application to bat throughout an absorbing morning session against tight bowling from the hosts.
Van Zyl moved to his half-century before surviving a decent lbw shout from Toby Roland-Jones in what was a probing opening spell from the former England seamer.
Brown at this stage was completely becalmed and he got the benefit of the doubt on another lbw appeal when he had made just 18, Ethan Bamber the unlucky bowler. Just 83 runs came in that first session, but even so it meant Sussex were already more than 100 in front.
The new ball was available almost immediately on the resumption, but instead of bringing much-needed wickets for the hosts it was the signal for a furious assault from Brown, who unfurled a series of glorious shots.
It set up a race between the two batsmen as to who could get to three figures first. In the event van Zyl just prevailed, reaching the landmark from 233 balls with 12 fours.
Not long afterwards it was Brown's turn, 16 boundaries getting him to the century almost 100 balls quicker than his team-mate. His second fifty came up in 41 balls in a passage of play which saw 81 runs in the 55 minutes after lunch.
It was 2:35pm by the time Middlesex got their first success of the day, Brown slogging across a straight one from Roland-Jones and losing his off bail.
Two more wickets fell before tea, David Weise pulling a short one from James Harris to Roland-Jones in the deep, before Chris Jordan played on to give Bamber his first scalp of the innings.
Van Zyl remained and any hopes of running through the tail were hindered by staunch support from Will Beer, who ensured maximum batting points were secured before tea. The pair batted deep into the final session taking the eighth wicket stand to 89 before van Zyl's epic vigil ended when he was trapped in front by Harris.
There was still time for Beer to make 50 and so equal his career-best, before Harris struck for a fourth time to scatter Ollie Robinson's stumps, provoking Jason Gillespie's men to declare 343 to the good.