Kyren Wilson moved into a 5-3 lead over world number one Judd Trump in their World Championship quarter-final clash at the Crucible.
Wilson drew first blood with a break of 74 in the opener but defending champion Trump responded with scores of 85 and 76 in the next two frames to edge ahead.
A run of three successive frames then helped Wilson build a 4-2 lead before Trump reduced the deficit by taking the seventh.
However, Wilson was able to restore his two-frame lead with a break of 79 just before the interval to leave Trump tailing after the first session for the third successive match.
Selby edged a marathon 58-minute opening frame, the longest in this year's tournament, and then reeled off the next four frames to race 5-0 in front.
However, Robertson hauled himself back into the match by winning the final three frames of the session, recording breaks of 83, 66 and 65.