After a very strong return to late-nights, it now appears that Jay Leno is going to be in for a battle with David Letterman for ”night owl” television supremacy.

The Nielsen Co. says Leno averaged 5.6 million viewers last week, his first back at NBC’s “The Tonight Show.”  Letterman had 3.7 million at CBS.

Leno took advantage of curiosity over his return and a strong lineup of guests, including Olympic stars and Sarah Palin. It was a half-million more than he averaged his last season at “Tonight.”

However; it’s been a lot closer the second week, just ending.  Through the first four nights, Leno was averaging 4.3 million viewers while Letterman is nipping at his heels with 4 million.

Considering Leno usually beat Letterman by a million and a half during their last season of competition, that’s a sign of erosion for NBC’s late-night franchise.

Letterman consistently whupped up on Conan O’Brien after the C-man took over last year.