Williams finished with 61 yards on 15 carries. Matt Moore completed 19 of 27 passes for 208 yards for the Panthers (6-8), who have won two of their last three games.
Matt Hasselbeck managed 27-of-41 passing for 274 yards and a touchdown for Seattle (9-5), which already owns the NFC West title.
Shaun Alexander gained just 17 rushing yards on seven carries for the Seahawks.
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