Williams played 79 minutes for the Dragons to help his side fight back to earn a 1-1 draw against the USA in Group B on Monday.
The 21-year-old defender was left in tears on the pitch at the full-time whistle and later revealed on social media that his mum told the news of his grandfather's passing on Sunday night.
“Yesterday was the toughest news I’ve ever had to face and that was listening to my mum tell me my grandad past (sic) away last night,” Williams wrote on Instagram.
“He’s been everywhere around the world to watch me play football from when I first started playing at Liverpool at 6 years old.
“He’d never tell me if I had a good game because he always said I have to get better and better each day and that’s why I am where I am now!
“So I dedicate this game all to him because I know he’s up there watching down on me very proudly.
“For everyone please cherish you have with loved ones because you never know when it can get taken away from you.”
On the pitch, a late second-half penalty from talisman Gareth Bale was enough to salvage a point for Wales after a sluggish opening period.
Robert Page’s side next face Iran, who were thrashed 6-2 by England in their opener, in Al Rayyan on Friday morning.