Following his return from injury, Williams was an automatic selection for the 2007 Rugby World Cup, especially as two other locks, James Ryan and Jason Eaton were unavailable owing to long-term injuries and Keith Robinson had suffered a calf strain during the Tri Nations and was a doubtful starter, at least for the first pool game. Robinson suffered a recurrence of his calf strain during pre-tournament training, which left Williams and Chris Jack to share the locking duties during the early games. Williams started in New Zealand's first three games, against , and Scotland. He was replaced for the last 10 minutes of the Italy game, his place being taken by number eight Rodney So'oialo. So'oialo again replaced him for the last 25 minutes of the game against Portugal, but not before he had scored his fifth international try. He scored again in the game against Scotland, which he started alongside Reuben Thorne, being replaced for the last 15 minutes by Chris Jack. Robinson was fit again for New Zealand's final pool game, against , and he and Thorne started as locks, with Jack on the bench; Williams was rested for this game.