Essex 180 (Walter 64, Clark 3-33, Worrall 3-40) and 21 for 2 need another 340 runs to beat Surrey 262 and 278 (Patel 107, Smith 70)
A brilliant 107 from Ryan Patel, necessarily watchful at first but buccaneering by its end, built on Jamie Smith's classy 70 and enabled Surrey to reach a second innings 278 and set Essex a formidable 361 to win the top-of-the-table clash at the Kia Oval.
By stumps, they were 21 for two with Nick Browne edging Jordan Clark to keeper Ben Foakes - via a push-up from a diving Smith at third slip - to depart for nine, and nightwatchman Eathan Bosch bowled for one as he shouldered arms in Clark's next over. Dean Elgar is 11 not out.
Patel thumped paceman Shane Snater for two legside sixes in an over and soon afterwards hit Simon Harmer's off spin into the Bedser Stand during a 125-ball knock that also featured 15 fours - the 14th of them a stunning straight drive off Matt Critchley to complete the 26-year-old's fourth first-class hundred.
Smith had earlier lit up a stop-start day as, for the second time in the match, he made sure Division One leaders Surrey stayed in control of a vital fixture against second-placed Essex.
With 35 overs ultimately lost to bad light and rain, across an infuriating five separate interruptions in the first half of the day, Surrey stretched their 126-run overnight lead to 360 as first Smith and then a 65-run seventh wicket partnership between Patel and Clark denied Essex a way back into the game.
Patel's superb late hitting then saw him plunder the bulk of the 85 runs Surrey managed to score for their last two wickets. Overall, indeed, it was a great effort by Surrey, who began day three on 44 for three, and the only consolation for Essex was seamer Jamie Porter claiming his 500th first-class wicket by having Clark caught at third man, uppercutting, for 30.
Leading Essex by 12 points coming into the match, Surrey would stretch that gap to 29 if they go on to secure victory on day four and - if that does happen - they would chiefly have Patel and 23-year-old Smith to thank for a sixth win from nine Vitality County Championship outings so far this season.
Smith's opening day 100 did much to take Surrey to 262 in their first innings and, resuming on 11, the player selected at the weekend to be England's new Test wicketkeeper-batsman later this month again showed his sublime strokemaking skills even in such bowler-friendly conditions and with the need to keep starting again and again after each break for bad weather.
After an initial 30-minute passage of play, in which Foakes was dropped on nine at first slip off Porter as Surrey edged to 63 for three, there were three mini-sessions of 10 balls, one ball, and then another 10 balls, either side of an early lunch.
More rain then followed a 27-ball session, driving the players off yet again at 2.12pm, but after a fifth re-start of the day at 2.38pm there was at last a prolonged period of play - broken only by the tea interval - until stumps.
By then Smith had moved smoothly past fifty, despite seeing Foakes fall for 13 when he edged Snater and was smartly held above his head at second slip by Harmer.
Left-hander Sai Sudharsan, the India international, helped Smith to add 45 for the fifth wicket, taking Surrey's overall lead beyond 200, but it was Smith who was the main aggressor with a booming extra cover drive off Snater and a gorgeous on-drive off Porter perhaps the pick of his eventual 11 fours.
And Smith, who came in on 25 for two, had scored 70 of Surrey's next 98 runs when he greeted left-arm seamer Paul Walter's first ball by pulling it straight to deep square leg in the 38th over.
When Sudharsan fell for 12 in the next over, thin-edging Bosch behind, Surrey were suddenly 127 for six and Essex knew they had probably their last chance to stay in the contest.
Patel and Clark, however, both batted with grit and no little skill to defy the Essex seamers for almost 20 overs, and Surrey's lead had reached 274 before Porter's dismissal of Clark was quickly followed by Bosch bowling Tom Lawes for a first-ball duck.
That was the signal for Patel, who had survived a caught and bowled chance to Bosch on 40, to press hard on the accelerator and although James Taylor (3) was bowled sweeping at Harmer he found in last man Dan Worrall a steady partner while a further 43 runs were added in, by now, bright sunshine and until Patel was bowled swinging at Critchley.
