Lowestoft Town are 90 minutes from Conference football after grinding out a 1-0 victory in Wednesday night's Ryman League Premier Division play-off semi-final.
Chris Henderson's bundled effort just 10 minutes from time proved enough as East Thurrock regularly threatened to silence the 1,200 fans inside Crown Meadow.
But the Town number 10's late strike did the trick, booking a place in Monday's play-off final – also at Crown Meadow against Concord Rangers, after their surprise 2-1 extra-time win at Wealdstone in the other semi-final.
Thurrock's best chance of the opening half came as early as four minutes in, Ryan Sammons' cross headed narrowly over from close in by David Bryant after patient build-up.
The visitors were already looking bright on the break, as Lowestoft tried to force the issue but all too often found their direct forays breaking down in the final third.
Chris Henderson had a wonderful chance to change that on 12 minutes as goalkeeper Andy Reynold's punt forward caught Sheehan napping, but the striker could only pull his shot agonisingly wide.
From there the game got feisty. Scott Mitchell and Ross Parmenter tangled after an aerial challenge, and Erkan Okay picked up a deserved booking for a late tackle on the same United man – one that could arguably have been worse from referee Oleksandr Saliy.
Dan Gleeson's superb interception was all that denied the unusually quiet Sam Collins lurking at the far post, while Bryant took too long over another chance as Thurrock grew into the game.
But Town responded, albeit only producing a stunning 25-yard Adam Smith volley that ran out of dipping time and a half-hearted penalty appeal from Tom Stephen and Henderson's tangled just inside the area.
A much improved Lowestoft came out with noticeably renewed purpose after the break, but it was still Thurrock carving out genuine chances.
Kye Ruel's brilliant break down the right deserved more than Higgins' limp effort to Reynolds, who then produced a superb point-blank save to tip over Collins' far-post header, while Bryant headed another cross wide when he should have done better.
But Joe Francis' introduction provided an initial spark as Town tried to force home a scramble from Adam Smith's corner, and that spark caught fire on 80 minutes as a Town free-kick brought an almighty goal-mouth scramble – Chris Henderson drilling high and home to send Crown Meadow into raptures.
And the Blues saw out United's desperate final and bad tempered flings, as well as almost six minutes of added time, to take them within one game of promotion.
• Town (4-4-2): Reynolds; Gleeson, Williams, Crane, A Smith; L Smith, Okay (Francis 67), Fisk, Mitchell; Araba, Henderson (Frew 90+3). Unused subs: Plummer, Benjamin. Booking: Okay 21, Araba 90+2. Goal: Henderson 80.
• Thurrock (4-4-2): Justham; Sammons, Sheehan, Peddie, Stephen; Ruel, Clifton, Parmenter (Lawson 86), Collins; Higgins, Bryant (Menga 76). Unused subs: Barnes, Wood, Keith. Booking: Collins 61, Sammons 82, Sheehan 86.
• Referee: Oleksandr Saliy; Attendance: 1,182
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