"I don't stand a chance of winning the title if I play like that again," said Higgins, who next meets Anthony Hamilton or Jason Ferguson.
In fact, Clark let the 22-year-old Higgins off the hook when he missed a final black in frame seven after looking set to cut his deficit in half.
Higgins' fellow Scot Jamie Burnett had earlier reached the last eight of a ranking tournament for the first time in his career. The 22-year- old from Hamilton recovered from 3-1 and 4-2 down to beat the 1991 World Matchplay champion, Gary Wilkinson from Nottinghamshire, but he said: "I didn't really deserve to win.
"Gary split the pack on a number of occasions and was unlucky not to land on anything. But I haven't always had the rub of the green in the past so I am not complaining too much."
Burnett's reward is a guaranteed pay day of pounds 7,500, plus a match with the world champion, Ken Doherty, or Pakistan's Shokat Ali.Reuse content