ImagoWorld number one Selby, who had kicked off his campaign with victories over Lucky Vatnani and Rory Mcleod, met his match in Harold in the fifth round at the South West Snooker Academy.
Harold, ranked 47 places below Selby, won 4-2 to secure a place in the last 16, where he will now play Andrew Higginson.
Higginson reached the sixth round thanks to a 4-2 win over Jamie Cope, having already disposed of John Astley and Joel Walker.
Stephen Maguire, Andrew Pagett, Alan McManus, Dechewat Poomjaeng and James Wattana also sealed places in the last 16.
Barry Hawkins edged a late-night battle with Michael Holt 4-2 to complete the round of 16 line-up for this half of the draw.
Round three results:
(match number in brackets)
59) Michael White 4-2 Tom Maxfield
60) Alan McManus 4-0 Chen Zhe
61) Dominic Dale 4-1 Martin O'Donnell
62) Peter Lines 4-0 Phil O'Kane
63) Dechawat Poomjaeng 4-1 Thepchaiya Un-Nooh
64) Jimmy Robertson v Michael Leslie
65) Anthony McGill 4-2 Jamie O'Neill
66) Xiao Guodong 4-0 Shane Castle
51) Mark Selby 4-0 Lucky Vatnani
52) Rory McLeod 4-2 Callum Downing
53) Dave Harold 4-2 Daniel Wells
54) Gerard Greene 2-4 Sydney Wilson
55) Kurt Maflin 0-4 Joel Walker
56) Andrew Higginson 4-3 John Astley
57) Jamie Cope 4-3 Reanne Evans
58) Anthony Hamilton 4-2 Michael Wild
67) Ricky Walden 4-0 Ian Glover
68) Barry Hawkins 4-2 Stuart Carrington
69) Nigel Bond 1-4 Pankaj Advani
70) Michael Holt 4-1 Billy Castle
71) James Wattana 4-0 Michael Wasley
72) Shaun Murphy w/o Mohamed Khairy
73) Andy Hicks 4-0 Toby Simpson
74) Barry Pinches 3-4 Thanawat Tirapongpaiboon
75) Robert Milkins 4-2 Ben Judge
76) Mark Joyce 4-1 Ben Harrison
77) Passakorn Suwannawat 2-4 Andrew Pagett
78) Yu De Lu 4-2 Lee Page
79) Marco Fu 1-4 Gary Wilson
80) Liu Chuang 4-2 Saqib Nasir
81) Stephen Maguire 4-0 Eden Sharav
82) Adam Duffy 2-4 Paul Davison
Round four results:
115) Mark Selby 4-2 Rory McLeod
116) Dave Harold 4-3 Sydney Wilson
117) Joel Walker 0-4 Andrew Higginson
118) Jamie Cope 4-1 Anthony Hamilton
119) Michael White 0-4 Alan McManus
120) Dominic Dale 4-3 Peter Lines
121) Dechewat Poomjaeng 4-0 Jimmy Robertson
122) Anthony McGill 3-4 Xiao Guodong
123) Ricky Walden 2-4 Barry Hawkins
124) Pankaj Advani 0-4 Michael Holt
125) James Wattana 4-2 Shaun Murphy
126) Andy Hicks 4-3 Thanawat Tirapongpaiboon
127) Robert Milkins 0-4 Mark Joyce
128) Andrew Pagett 4-2 Yu Delu
129) Gary Wilson 4-0 Liu Chuang
130) Stephen Maguire 4-1 Paul Davison
Round five results:
147) Mark Selby 2-4 Dave Harold
148) Andrew Higginson 4-2 Jamie Cope
149) Alan McManus 4-3 Dominic Dale
150) Dechewat Poomjaeng 4-2 Xiao Guodong
151) Barry Hawkins 4-2 Michael Holt
152) James Wattana 4-2 Andy Hicks
153) Mark Joyce 0-4 Andrew Pagett
154) Gary Wilson 0-4 Stephen Maguire
