Luke Wells made a career-best 208 as Sussex turned the screw on Surrey to leave the hosts needing to hang on for a draw going into the final day.
Wells continued to dominate proceedings as he completed a nine-hour double century to help Sussex to 526 all out in the first innings to lead Surrey by 175 with still 17 overs left in day three.
Rory Burns went 15 minutes into Surrey's reply, but new skipper Graeme Smith (43) and first innings century-maker Gary Wilson (14) survived through to stumps to take their side into the fourth day on 66-1 - still 109 runs behind.
The Sussex batsmen barely put a foot wrong in the first session as the visitors resumed on their overnight score of 204-2.
Wells and his skipper Ed Joyce saw off the second new ball before 22-year-old Wells played the shot of the day to reach his 150 - caressing a cover drive against Zander de Bruyn to the extra cover ropes.
Joyce went just two runs short of his century on the stroke of lunch to end their partnership - as he lost his off stump when looking to steer a Gareth Batty delivery down to third man.
England keeper Matt Prior increased the tempo in the mid-session launching into an all-out attack to post a 35-ball 50 as he and Wells added 91 in 15 overs.
Downed early on by Vikram Solanki at short mid-wicket, Prior raced to 62 from 45 balls (46 of them in boundaries) before toe-ending an attempted lofted cover drive against Jade Dernbach into the hands of Smith at wide mid-off.
After surpassing his career-best 174 scored at Hove against Yorkshire in 2011, Wells cantered through to his maiden double hundred with a single to deep extra cover against Vikram Solanki.
Wells' 200 included 25 fours and a six and came off 406 balls and is the highest individual score in the history of this fixture at The Oval, with his innings finally being ended by Solanki when he was bowled looking to hit an 'inside-out' lofted cover drive.