by Andrew Reid and Yahoo UK staff
Williams was one of the standouts for the Welsh during a tense Group B clash that saw Gareth Bale's late penalty rescue a draw against the Americans at a raucous Ahmad Bin Ali Stadium.
The 21-year-old Nottingham Forest fullback could be seen in tears on several occasions, with Williams later revealing it was as a result of his family bereavement.
Williams revealed after the final whistle that he had only found out from his mother on the eve of the match that his grandfather had passed away, describing it as the "toughest news I’ve ever had to face".
The Wales defender could be seen walking around the pitch in tears after fulltime, as teammates and coaching staff consoled the grief-stricken player.
At one point Williams spotted a camera pointing in his direction and said: “That was for you grandad.”
He later wrote on Twitter: “Yesterday was the toughest news I’ve ever had to face and that was listening to my mum tell me my grandad past away last night.
“To go from crying all day to start in a World Cup game was extremely tough but I got through it from the support of my team mates and family.”
He added: "So I dedicate that game and an important point to him."
Williams - who played 79 minutes before being substituted after injuring his back in an awkward challenge - was flooded with messages of love and support on social media.
Watch: Wales fans react to1-1 draw in Doha
Harry Wilson, the Wales star who played alongside Williams against the USA said: "Made him proud brother."
Williams' former Liverpool team-mate Harvey Elliott posted: "Made him proud my bro! Keep strong."
Young Blackpool star Rhys Williams said: "Doing him proud brother."
there were other messages of support from footballers including Leighton Clarkson, Nathaniel Phillips, Owen Beck plus the Welsh FA.
Nottingham Forest Fan site 'Forest Watch' said: "Neco Williams lost his grandfather last night. Tonight he played the first World Cup meeting of his young career with Wales. A model of bravery!":
Cardiff City sports journalist Glen Williams said: "Neco Williams on Instagram. Fair play to him for putting a shift in like that tonight in the circumstances. What a warrior. We are lucky to have him."
TV presenter Adam Smithy said: "Neco Williams found out his Grandad passed away LAST NIGHT! Today he bossed it for Wales in his 1st ever World Cup game. His family should be VERY PROUD."
Gareth Bale comes to rescue
Wales’ first World Cup match for 64 years looked like ending in defeat after Timothy Weah rewarded the Americans’ first-half dominance with a clinical 36th-minute strike at the Ahmad bin Ali Stadium in Doha.
But Bale was felled in the box eight minutes from time by Tim Ream and picked himself up to smash home his 41st goal for Wales from the penalty spot.
Bale said: “The goal was exactly what we needed. It didn’t matter who scored it and it gave us a foothold to try and go and win it.
“I felt that we played very well in the second half, so personally, it was great to get the goal. But like I said, the most important thing was that the team scored.
“In terms of the huddle after the game, it’s something that we do, especially in major tournaments.
“We just need to keep the momentum going now, recover and go again.”
Wales’ next World Cup game is against Iran, who were thrashed 6-2 by England earlier in the day, on Friday.