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.
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.
In the crowd in Sheffield was Leeds United manager Garry Monk.
Reigning champion Mark Selby moved closer to a place in the semi-finals after storming to a dominant 6-2 lead over Marco Fu.
The world number one was at his devastating best as he reeled off the first five frames of the match before Fu hit back by taking two of the last three of the afternoon session to give himself a glimmer of hope at the Crucible.
Selby took advantage of the weary-looking Hong Kong player - who needed a 13-11 win against Neil Robertson in a late finish on Monday evening to seal his place in the last eight - by racing out of the blocks.
The ‘Jester from Leicester’ took control of the opening frame and earned a 2-0 lead with a patient clearance of 45.
He extended his advantage further in the third with a break of 82 and took the next two courtesy of runs of 70 and 94.
Fu, who had not potted a ball in over an hour during Selby’s commanding spell, responded with successive frames to reduce the deficit to 5-2 after the world number eight finally got among the balls.
But Selby ensured he finished with a four-frame cushion thanks to a terrific snooker which took Fu four attempts to escape as he took a step closer to defending his title.
On the other table, Barry Hawkins took a 5-3 lead against Stephen Maguire, who would have been relieved to win the last two frames of the session and stay in touch.
Maguire raced out of the blocks to post an 81 break in the first frame but runs of 46 and 98 helped Hawkins move 3-1 ahead at the mid-session interval. The 38-year-old edged the fifth to extend his advantage and scores of 45 and 34 increased his lead even further to 5-1.
The Scot stopped Hawkins’ five-frame winning run by stealing a tight seventh frame on the black and crucially also won the last frame of the afternoon.
John Higgins needs two more frames for victory, leading Kyren Wilson 11-5 overnight.