CBS’ “Under the Dome” seems to be running out of gas as it nears the end of its second season, falling to a series low on Monday, a night that saw NBC’s “American Ninja Warrior” and Fox’s “MasterChef” log the top demo scores.

NBC and Fox tied as the night’s No. 1 broadcaster in 18-49, while CBS led the way in total viewers.

According to preliminary national estimates from Nielsen, “Under the Dome” averaged a 1.2 rating/4 share in adults 18-49 and 6.3 million viewers overall — down 14% (0.2) from last week (1.4), which had been its low. It remains summer’s top scripted series, and continues to be popular in DVR playback, but it’s clear that “Dome” has lost some of its heat this summer.

“Under the Dome” had a distant fourth-place lead-in among the Big Four, with a repeat “Two and a Half Men” settling for a 0.7/2 in 18-49 while drawing 4.3 million viewers overall in the 9:30 p.m. half-hour. Earlier, repeats of “The Big Bang Theory” (1.2/4 in 18-49, 6.0 million viewers overall) and “Mike & Molly” (0.8/2 in 18-49, 4.7 million viewers overall) were sandwiched by the “CBS Fall Preview” special at 8:30 p.m. (0.8/3 in 18-49, 4.7 million viewers overall).

Elsewhere, NBC’s “American Ninja Warrior” (1.9/6 in 18-49, 5.8 million viewers overall) tied as the night’s No. 1 series in 18-49, hitting a three-week high in the demo while drawing its largest overall audience in six weeks. Leading off the night, “Running Wild With Bear Grylls” (0.9/3 in 18-49, 3.9 million viewers overall) drew its smallest demo rating in five airings to date.

Fox’s “MasterChef” continues to do well leading off Monday (1.9/6 in 18-49, 5.4 million viewers overall), down a tenth from last week but tied as the night’s No. 1 series in 18-49. It was followed by “Hotel Hell” (1.3/4 in 18-49, 3.9 million viewers overall), which was down three tenths from last week’s season high.

At ABC, both “Bachelor in Paradise” (1.2/4 in 18-49, 4.7 million viewers overall) and the season finale of “Mistresses” (0.8/3 in 18-49, 3.5 million viewers overall) were down a tenth from their national scores of last week (which represented their low-water marks of the summer) but on par with last week’s prelims. “Mistresses” likely will rise to a 0.9 in the nationals, but that would still leave it down from last year’s 1.1 finale score. In overall viewers, the drama was up for a second straight week and will come in closer to last year’s tally (3.9 million).

CW’s “Whose Line Is It Anyway” aired a repeat at 8 (0.4/1 in 18-49, 1.4 million viewers overall), followed by its regularly scheduled 8:30 p.m. encore (0.4/1 in 18-49, 1.5 million viewers overall). “America’s Next Top Model” (0.4/1 in 18-49, 1.3 million viewers overall) matched the previous week’s demo rating while rising in total viewers.

Preliminary 18-49 averages for the night: NBC and Fox, 1.6/5; ABC and Univision, 1.1/3; CBS, 1.0/3; Telemundo, 0.6/2; CW, 0.4/1.

In total viewers: CBS, 5.4 million; NBC, 5.1 million; Fox, 4.7 million; ABC, 4.3 million; Univision, 2.8 million; CW and Telemundo, 1.4 million.