Each man amassed their only notable break of the contest in an even semi-session, with Welshman Williams’ 86 coming immediately after the Hong Kong-born Fu’s 74.
But after the two emerged from their mid-session interval level at 2-2, Fu seized control and grafted his way to three consecutive frames to ensure a last-16 fixture with Ding Junhui on Thursday.
Stuart Bingham hit a 126 total clearance on his way to a comfortable 5-2 win over local amateur wild card qualifier Zhao Xintong which sets up a round two confrontation with four-times world champion John Higgins, who fought off a game Alan McManus 5-3.
Both Scotsmen hit big in an entertaining affair, beginning with a Higgins 51 and McManus 68 for 1-1.
Higgins took in a run to 62 as he claimed the next two frames for a 3-1 lead, and despite two McManus half-tons to pull the fifth back, his decorated opponent would remain one step ahead.
A 61 saw the world number five regain his two-frame cushion and though McManus pulled another back, it would be his last as Higgins broke to 68 to advance.
Finally, Stephen Maguire bested Marcus Campbell 5-3 to complete the round of 16 bracket, where the Welsh Open champion will play Ricky Walden.
All round two contests will be played on Thursday February 28, with the quarter-finals on Friday, semi-finals on Saturday and the final on Sunday over two sessions.
Haikou World Open results – Wednesday session two
Stuart Bingham 5-2 Zhao Xintong
Mark Williams 2-5 Marco Fu
John Higgins 5-3 Alan McManus
Stephen Maguire 5-3 Marcus Campbell