Williams had a hand in each of his team's two first-half tries, which proved pivotal in the end when the Blues launched a strong second-half comeback.
Returning to the Chiefs after spending the last two seasons playing rugby league in Australia, Williams helped set up tries for James Lowe and Bryce Heem.
"It was a very tough game," said Chief skipper Matt Symons. "It was good to get away with the win but there's still plenty of things to work on."
Wallabies star Tomane treble puts Brumbies in Super Rugby semis
The Chiefs, who won back to back Super Rugby titles in 2012 and 2013, opened up a 20-6 lead in the first half before the Blues came back strongly in the second term.
Although they did not score a try, the Auckland-based Blues kept chipping away the lead by kicking six penalties, all from Ihaia West.
"There were a lot of positives we can take out of this," Blues captain Jerome Kaino sad. "The forwards went bout their work well and the backs did some good things too."
Draw puts Stormers in Super 15 play-offs
Israel Folau hat-trick seals play-off place for rampant NSW Waratahs