Williams was sent off for striking Springbok flanker Heinrich Brussow - with the former Sale back admitting to the charges.
Judicial officer Christopher Quinlan QC, though, determined that the offence was "at the lower end of the scale of seriousness".
A Rugby World Cup statement read: "The judicial officer found that there were compelling on-field and/or off-field mitigating features and a complete absence of on-field and/or off-field aggravating features.
"In particular he determined that the strike was an open-handed push, the contact was not heavy and there was no adverse effect on the victim player or the match.
"Therefore, in accordance with disciplinary clause 12.5 the judicial officer determined that no further sanction was necessary.
"Williams is free to play with immediate effect."
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