Seeking to sink “Sin City” from its box office perch this weekend are 20th Century Fox’s “Fever Pitch,” at 3,267 theaters, and Paramount’s “Sahara,” which bows in 3,148 venues. Though there’s no clear favorite for the frame’s B.O. crown, the Jimmy Fallon-Drew Barrymore romantic comedy may have a slight edge. Tracking shows the film drawing audience interest evenly (if moderately), and relationship comedies have been playing well in the early months of 2005, demonstrated by the monster hauls for “Meet the Fockers” and “Hitch.” “Pitch” may not be in their league, but two weeks ago “Guess Who” bowed better than expected with $20.7 million. And last fall, Fallon’s first bigscreen outing in “Taxi” opened in October with $12 million. Industry estimates put “Pitch” somewhere in between. “Sahara,” with a $100-million-plus budget, has a fate less clear. While similarly themed hunt film “National Treasure” came out of nowhere at Thanksgiving with a $35.1 million debut, last spring “Hidalgo” had trouble mustering its $18.8 million opening. Older males have been the demo showing most interest in “Sahara,” while younger auds have yet to show much spark. As with “Pitch,” “Sahara” may end up opening in the mid-teens, which is where “Sin City” would end up if it drops 50% from its opening weekend of $29.1 million.