More people watched a substitute for Brian Williams on NBC’s “Nightly News” than they did Diane Sawyer’s final “World News” broadcast last week on ABC, but advertisers may not be as impressed.

That’s because the ABC evening newscast was last week watched by more viewers in the demographic most coveted by news-program sponsors – people between 25 and 54 – than NBC’s. It’s true, NBC’s “Nightly News” continues to be the most-watched evening news broadcast on television. And season to date, it has also captured the biggest share of both people between the ages of 25 to 54 and 18 to 49. But for the last four weeks –  and for eight of the past nine – ABC’s “World News” has captured a greater slice of the 25-to-54 audience.

For the week of Aug. 25, ABC’s “World News,” anchored by Sawyer for the first three days of the week, attracted about 1.94 million viewers between 25 and 54, compared with about 1.67 million for NBC’s “Nightly News” and nearly 1.45 million for CBS’ “Evening News.” According to ABC, “World News” outperformed NBC “Nightly News  by 271,000 news demo viewers to match its largest margin of victory in more than seven years, since the week of July 2, 2007.

Due to the “Emmy Awards,” “NBC Nightly News” was re-titled on Monday, and the Monday-night broadcast was excluded from the program averages.  NBC’s weekly averages are based on four days, Tuesday through Friday.

Late-summer viewing patterns are not the most reliable, owing to consumer tendency to disrupt regular habits with vacations and entertainment. But ABC’s increasing ability to snare in-demo viewers is likely not going unnoticed. Critics have charged the Walt Disney network has snared younger viewers by speeding up the formats of its various news shows and offering a broader palette of stories, which are shortened, leaving less time for depth. At the same time, the numbers are likely to be followed closely by advertisers, who count on amassing viewers in that age range.

The skirmish is part of an ongoing battle that will only continue as ABC puts its faith in David Muir, a younger anchor, who has now taken over its “World News” in full. At 40, Muir is younger than CBS’ Scott Pelley and NBC’s Williams, as well as Sawyer – and the network’s faith in him may suggest it is eager to woo a new generation to its news products as the baby boom generation moves more steadily into retirement.

NBC’s “Nightly” is still the leader of the race, and not to be dismissed. Season-to-date, “Nightly” leads in total viewers and viewers between 25 and 54 and 18 and 49. The broadcast has increased its total viewership during that time by 6%, or 490,000 viewers, and has broadened its overall advantage over ABC by 28%.

Yet as the recent flip in rankings between ABC’s “Good Morning America’ and NBC”s “Today” has demonstrated, playing by the second-place handbook can undermine the first place holder’s position without much of a warning.