No. 3 Utah will visit USC this Saturday as the only top-four team in the AP poll to be an underdog against an unranked team since at least 1998, the good people at OddsShark.com were kind enough to discover for us.
No. 5 teams have been underdogs against unranked teams twice in the 18 seasons recorded by the OddsShark.com database (see here).
In October 2007, No. 5 Wisconsin was a 2.5-point dog at unranked Illinois and lost 31-26.
In November 2002, No. 5 Georgia got two points against unranked Florida for a neutral site game in Jacksonville and fell 20-13.