Middlesex 485 (Simpson 109, Roland-Jones 85, Malan 64, Robson 62, Currie 6-93) trail Sussex 523 (Pujara 231, Alsop 135, Helm 5-109) by 138 runs
The redoubtable gloveman reached three figures for the ninth time in his career in an almost five-hour vigil as the hosts recovered from the perils of 137 for 4 to reach 485 all out on the stroke of stumps.
Simpson (109) shared stands of 130 with debutant Pieter Malan (64) and 90 with Toby Roland-Jones (85) amid the fightback.
All this should not overshadow a magical debut for Sussex seamer Brad Currie, who produced a bewitching spell of four for 16 in the morning session to rip out Middlesex’s top order. The 23-year-old later returned to claim two more scalps for figures of six for 93.
The hosts resumed 103 without loss and when Sam Robson (62) creamed the first ball of the day through the covers it suggested a continuation of the run-fest.
However, in perfect swing conditions, Currie’s devastating spell threatened to change the entire landscape of the game.
The younger sibling of Scott Currie of Hampshire swung the ball with impunity and found that fraction of nibble which creates uncertainty, so threatening both the inside and outside edge of the bat.
Reward came quickly as his in-swinger beat Mark Stoneman’s inside edge to trap him lbw without adding to his overnight score.
It wasn’t long before Steve Eskinazi, back from England Lions’ duty became his next victim, an outside edge flying through to Oli Carter who clung on at the second attempt.
Adrenaline now coursing through his veins, Currie’s next two wickets came in the space of four balls, Former England opener Robson having his off-stump pegged back by one which beat the outside edge, before Max Holden was castled too via the inside of the bat. The hosts were in disarray at 137 for 4, leaving Malan and Simpson to re-entrench.
Malan, finally in the UK after his well-publicised visa issues, played confidently from the off, driving well on both sides of the wicket, while Simpson, one ill-advised sweep shot to the spin of Archie Lenham on the stroke of lunch apart looked his normal dependable self
The hosts prospered through the early afternoon, but the minute Poole-born Currie returned Malan was given a life when Carter spilt an edge, the resulting single taking the batter to 50.
And when Simpson followed his partner to the half-century mark from 137 balls Middlesex appeared to be inching towards safety.
However, in the shadows of the tea interval Greek international Aristides Karvelas got a debut wicket with one which lifted and left Malan, though the right-hander looked aggrieved to be given out caught behind and replays suggested he may have cause.
When teenager Danial Ibrahim had Luke Hollman well caught in the gully by Tom Clark from a full-bloodied square cut soon after the resumption, Middlesex were still 82 short of the follow-on.
By now the floodlights were on, but Simpson and Roland-Jones launched a counter-attack amid the gloom, reviving memories of their century-stand against Glamorgan back in April.
Roland-Jones showed controlled aggression striking the ball hard and straight, while Simpson timed the ball sweetly against a tiring attack.
A hard-run two to mid-off carried Simpson to his century in a shade over four and three-quarter hours and by the time Delray Rawlins bowled him soon afterwards the follow-on had been saved.
Currie though completed his day to remember by ending Roland-Jones’ swashbuckling innings before scattering Umesh Yadav’s stumps, but barring something extraordinary on day four this game seems destined for a draw.