NBC’s Brian Williams problem has developed into a problem involving Brian Williams’ absence.

ABC’s “World News Tonight,” anchored by David Muir, was the most-watched evening newscast for the five days between March 30 and April 3, according to Nielsen, marking the first time since September 7, 2009, that NBC’s “Nightly News” has not held that distinction, and the first time NBC has lost the most-watched title to ABC since Brian Williams was suspended from anchor duties on the show.

ABC’s newscast was also the most watched in the period by viewers between 25 and 54, the audience most coveted by advertisers in news programming. ABC’s newscast has edged out NBC’s in that measure for four weeks in a row.

ABC’s victory comes while the NBC broadcast has been in flux. Williams, the longtime anchor of “Nightly News,” is serving a six-month suspension following the disclosure that he lied about the details of a 2003 reporting trip to Iraq. NBC News is investigating other potential falsifications by Williams, and there is one school of thought within NBCU that the anchor may not return to his post. Lester Holt has anchored the program in Williams’ absence.

But the Walt Disney-owned network’s margin of victor is thin. ABC’s “World News” won an average of 7,997,000 viewers during the period, compared with an average of 7,913,000 viewers for “Nightly” — a win encompassing just 84,000 viewers. Among viewers in the 25-to-54 demographic, ABC’s win was more substantial: “World News” won an average of 1,952,000 viewers, compared with NBC’s 1,791,000 viewers — a lead of 161,000.

“Lester Holt has led NBC Nightly News to great success over the past two months, and we continue to be pleased with his strong performance during this time,” NBC News said in a statement.” We’re proud of the caliber of our work and remain committed to delivering the highest quality broadcast for our audience every night.”

This week’s ratings tabulation marked the first time in several weeks that NBC’s numbers did not include a measure of overnight viewing from the reairing of “Nightly” in ten different markets in the wee hours of the morning. NBC had struck a deal with a passel of 10 stations owned by Gannett Co. to air “Nightly” between 2 and 4 a.m., much as it had been doing with the fourth hour of its morning-show “Today,” featuring Kathie Lee Gifford and Hoda Kotb. But after ABC raised an outcry on the issue, several advertisers insisted that their commercials not accompany the overnight airings and NBC was left without that boost to its numbers.

ABC’s evening-news ratings have been building for some time. Under Muir, the ABC newscast has gradually made inroads in the 25-to-54 demographic, but it has not truly had a victory among total viewers in years. ABC was believed to have won in total viewers for a brief time last October, until it was discovered that an error in tabulation by Nielsen made those results moot.

This battle is not decided, however. Season-to-date, “NBC Nightly News” remains the most-watched evening newscast in the U.S. as well as the most-watched by the adults 25-54 audience. Still, the margins are slim. “Nightly News” leads “World News” by 6% in total viewers and just 1% — a mere 22,000 viewers — among viewers between 25 and 54.