Mark Selby has crashed out of the Australian Open, after losing 5-3 to world number 29 Ken Doherty.
The Leicester potter could not emulate his comebacks over Joe Perry and Mark Davis after Doherty took a 4-1 lead.
Selby fought back to 4-3 and had a commanding lead in the eighth frame but Doherty cleared the table to book his semi-final place.
The victory means Doherty has reached a ranking semi-final for the first time since 2006.
Dubliner Doherty, 41, revealed he almost retired two years ago when his world ranking dropped as far as 55.
"I felt like packing it in at one stage when I never qualified for the Crucible," he said.
"I was in qualifying tournaments and only won one match all season and I thought it was the end of the road."
Doherty has now beaten two Top 10 players in the last two matches and says Welshman Williams' own return to form has been a motivating factor - but not as his fellow countryman Darren Clarke's Open victory last weekend.
"Mark's come back from the depths of despair like me and it shows you can do it," said Doherty. "But my inspiration was Darren Clarke winning The Open at the age of 42."
Doherty clinched his quarter-final victory over Leicester's Selby with a battling clearance that he found particularly pleasing.
"From 57 behind with only four reds left to win it was my best clearance as a professional.
"In 20 years I've made some good ones but to close out the match from that position that was the best," he said.
Shaun Murphy will play Stuart Bingham in the other semi, after they beat Matthew Selt and Mark Allen respectively.