Thai player Saengkham - who knocked out Mark Selby and then Ronnie O'Sullivan in previous rounds - made three half-century breaks to edge past Masters runner-up Williams after the veteran Welshman failed to convert an opportunity in the eighth frame to force a decider.
Thursday's other match of the evening session finished on the stroke of midnight as Northern Ireland's Mark Allen also progressed to the last four after a gritty 5-4 victory over Jack Lisowski, who had fought back with a fine break of 68 to take the penultimate frame which had looked set for a rerack.
Earlier on Thursday, Masters champion Judd Trump cruised into the last eight with a 4-0 win over Belgium's Luca Brecel while Xiao Guodong defeated Welshman Ryan Day 4-1.
Anthony McGill made a break of 131 as he saw off Sam Craigie without dropping a frame and Shaun Murphy made a century clearance as he beat Kyren Wilson 4-2 to also reach the quarter-finals.