Irishman Allen recovered from 3-1 down to win a deciding frame, making a 109 break to kickstart his comeback in the fifth.
Xiao Guodong continued the defence of his crown by ending the fairytale run of English teenager Stan Moody.
Moody beat Ding Junhui and Barry Hawkins to reach a maiden ranking quarter-final but the 18-year-old was no match for the 2024 champion, whose breaks of 94, 87 and 100 helped him wrap up a 5-2 win.
The Chinese will next face Mark Williams after the 50-year-old brushed aside Zhang Anda 5-1 to reach the last four of a ranking event for the 68th time in his storied career.
“I’ve got to pinch myself sometimes with what I’m doing,” Williams told www.wst.tv. “The peak of my powers was between 24 and 31. I’m 20 years past that and I’m still getting to semi-finals of tournaments. It is incredible when you sit back and think of it.”
John Higgins, Williams’ fellow member of the ‘Class of ’92’, was eliminated after a 5-3 loss to Gary Wilson, who will face Allen next.
