Captain Ed Joyce played a lone hand for Division One leaders Sussex with 101 as they were bowled out for 229 on the first day of the LV= County Championship match at the BrightonandHoveJobs.com County Ground.
Middlesex's decision to bowl first was vindicated and they strengthened their position in 13 overs of batting before stumps as openers Sam Robson and Dawid Malan took them to 44 without loss.
Earlier, they struggled to dislodge one of their former players as Joyce continued his superb form, passing 800 championship runs on his way to a second century of the season.
While his team-mates found it hard to combat a persevering attack on a helpful surface, Joyce did not offer a chance and always seemed to have time to play his shots.
He hit 13 fours and went to three figures in style, upper-cutting Gareth Berg over a well-populated slip cordon and the third-man boundary for six.
Shortly afterwards, he played on to Neil Dexter, who took a wicket with the second ball of a spell for the second time in an absorbing day.
Apart from Chris Nash, who made 40, Joyce lacked support as a succession of batsmen wasted promising starts by getting out when well set.
A pitch offering good pace and carry assisted the seamers throughout a hot day but openers Nash and Luke Wells negotiated 16.1 overs before Navjit Sandhu, a 21-year-old seamer from Isleworth, took his first championship wicket when the latter was caught by wicketkeeper John Simpson.
Berg struck two balls later when the in-form Mike Yardy was caught behind for a golden duck, giving the bowler three wickets in four balls having taken two in two when the sides met at Lord's in June.
Nash seemed to be prospering but drove at a length ball from Berg well outside off-stump which Robson caught at gully.
Joyce, in contrast, looked at ease from the start and, either side of lunch, added 41 in 17 overs with Rory Hamilton-Brown before the latter was bowled offering no shot.
Luke Wright was also guilty of poor shot selection, picking out short midwicket when he came down the pitch in off-spinner Ollie Rayner's second over.
The theme continued after tea when Tim Murtagh picked up Chris Jordan, who played on, and Steve Magoffin, edged to Simpson.
Berg ended the innings when he had James Anyon held at third slip to finish with 3-49.