His Sept. 14 debut attracted 18.4 million viewers. By the second week, however, the number of viewers had dipped as low as 5.1 million.
A fall repeat of CBS's "CSI-Miami" topped Leno on a Monday night; not a good sign because Leno vowed this summer to beat the competing networks whenever they aired reruns. Before that, the FX series "Sons of Anarchy" was the first cable show to beat Leno.
Then there's the so-called Leno effect that has some of NBC's more than 200 affiliates grumbling that the show's weak lead-in is eroding audiences for their 11 p.m. newscasts. O'Brien's and Fallon's numbers for their late-night shows are also down, ostensibly because of a weaker Leno.
Leno made it clear in a 2009 interview with Broadcasting & Cable magazine that he wasn't ready to concede the fight.
"I enjoy being the underdog," Leno said. "Do I enjoy the battle? Yes, I get a certain amount of satisfaction from pounding my head against the wall.
"Emotionally, I can take body shots all day long and that doesn't really bother me," he added.
But in the end, bowing out of the 10 p.m. race wasn't a choice left up to him.
ABC News' Luchina Fisher and Michael S. James contributed to this report.