Every frame of the morning saw a break of 50 or more, Ding taking a 3-0 lead before O'Sullivan levelled the session at 4-4, but the Chinese player pulled ahead in the evening session.
Ding took frame one with a 71 break and followed that up with a magnificent 128 clearance to double his advantage. It got better for the world number four as another run of 71 put him further ahead, with O'Sullivan barely getting to the table.
But the 41-year-old hit back with a 63 to finally get on the scoreboard before the mid-session interval and took the next two after the restart with runs of 99 and 65 to level the match.
Ding scraped the next on a respotted black despite a 69 break from the five-time champion, who finished off with a run of 59 to restore parity.
The pattern continued into the evening as Ding returned with a frame-winning 63 clearance in frame nine.
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O'Sullivan came from behind to win the next but was unable to repeat the dose in frame 11 after over-running a positional shot to the black.
Ding took that one and made it 7-5 at the interval. He then resumed with a 120 break and went on to stretch his lead to 10-5 with a 58 in frame 15.
O'Sullivan, though, produced a defiant rapid-fire 104 in the final frame to give himself a chance.
If the closing century put that match in the balance, though, John Higgins looked to have broken the challenge of Kyren Wilson on the other table.
Wilson had levelled from 2-0 down in the morning, helped by a 92 break, but damaged his cue tip and, after a brief stoppage for repairs, saw four-time Crucible champion Higgins open up a 5-3 lead with breaks of 62 and 59.
The Scot began the evening with a 129 and, after the next two were shared, added breaks of 74 and 135 either side of the interval to lead 9-4.
Wilson responded well with a 97 - the spirit shown by the 'Warrior' would have impressed watching former England footballer Paul Ince.
Higgins, though, won the last two to leave himself two frames from victory at 11-5.
Quarter-Finals (best of 25 frames)
Ronnie O'Sullivan (12) 6-10 Ding Junhui (4) Frame scores: 0-79 (71), 12-128 (128), 7-77 (71), 79-11 (63), 99-0 (99), 102-16 (65), 69-76 (O'Sullivan 69) *respotted black, 19-107 (59), 40-87 (63), 84-42 (84), 31-64 (64), 56-82 (65), 0-120 (120), 35-68 (59), 0-87 (56), 104-0 (104)
Kyren Wilson (14) 5-11 John Higgins (6) Frame scores: 1-72, 5-94 (60), 67-31, 92-0 (92), 36-53, 62-15 (61), 17-63 (62), 9-67 (59) , 0-129 (129), 67-45, 86-42, 0-74 (74), 0-135 (135), 97-0 (97), 45-71, 4-71
Barry Hawkins (7) 5-3 Stephen Maguire Frame scores: 25-82 (81), 88-40, 65-41, 98-4 (98), 71-43, 79-27, 57-68, 21-66
Mark Selby (1) 6-2 Marco Fu (8) Frame scores: 71-0, 84-36, 82-0 (82), 97-0 (70), 94-0 (94), 17-77, 41-77 (60), 75-42
Fast Results
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