The world number 52 was beaten 6-3 by 38th-ranked Rory McLeod in the penultimate round this afternoon (Thursday), missing out on a final round clash against Robert Milkins.
Robertson had beaten McLeod on two previous occasions this year and got off to a good start this time as a 98 break gave him the opening frame 98-27.
But McLeod cruised through the second 84-1 helped by a 48 and the third 83-5 with a best effort of 36. Robertson got back on level terms by winning the fourth 94-35 courtesy of an 88 break.
Robertson began frame five with a 28 only for McLeod to clinch it 68-28 aided by a 46. A 39 helped McLeod bag the sixth 72-8 and he also landed the seventh 61-24 for a 5-2 lead.
Former world amateur champion Robertson, 26, recovered from an identical position to beat Michael Wasley 6-5 in the previous round yesterday and gave himself hope of another remarkable comeback by winning the eighth 58-44.
That hope was swiftly extinguished, however, as McLeod took the ninth 71-19 with all his points coming in a single visit.