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 (pictured) 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.
Click here for full results, schedule and draw details.
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.
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
