In the final match before he begins the defence of his world title at the Crucible Theatre in Sheffield on April 15, Selby proved why he is the game's number one with a performance of true grit against a valiant Williams.
Williams needed to win the title to sneak into the top 16 and avoid having to qualify for the World Championship, but came up agonisingly short.
The defeat means he will have to negotiate three qualifying rounds to reach the last 32 at the Crucible, but he will be confident about his prospects after producing some of his very best stuff to reach the final.
It is Selby's fourth ranking title of the season, and he collects a cheque for £85,000 to extend his lead at the top of the rankings.
Resuming 5-4 ahead after the first session, Williams won three of the first four frames of the evening session to lead 7-6 boosted by breaks of 68, 65 and 81.
Selby restored parity at 7-7 with a run of 95 only for his fellow double world champion Williams to pinch a tense 15th frame on the pink for an 8-7 lead.
It was to be his last morsel of success as Selby found another gear with breaks of 70, 124 and a dominant performance in the 18th frame enough to secure him the China Open title for a second time after winning in Beijing two years ago.
There was to be no first ranking event win for Williams since the 2011 German Masters with his fellow Welshman Ryan Day assured of an automatic place at the Crucible as the game's 16th ranked player.
Read the full article on eurosport.co.uk: Selby edges out valiant Williams to win China Open