It may have received nothing but disparaging reviews after a preview outing during the Winter Olympics' finale a week ago, but the Jerry Seinfeld-produced The Marriage Ref got off to a solid start in the ratings Thursday night at 10 00 p.m., the hour recently vacated by Jay Leno. The NBC show drew 7.71 million viewers, only about half the 14.78 million who tuned in to The Mentalist on CBS. But it edged out The Mentalist among viewers 18-49, the key demographic group. NBC also performed well with that demo in the 9 00 p.m. hour, topping both ABC's Grey's Anatomy and CBS's Csi Crime Scene Investigation. Nevertheless, the top-rated show of the evening -- by far -- was Fox's American Idol , which rarely performs as well on Thursdays as it does on Tuesdays and Wednesdays, but nevertheless raked in 20.71 million viewers. On Friday, NBC also produced strong ratings in the 10 00 p.m. hour (and better-than-decent ratings in the 9 00 hour as well). With far better numbers than Leno ever gave Conan O'Brien as a lead-in, the Tonight Show easily topped David Letterman during his first week back in his old time period.