As the United States leads the Rio Olympics 2016 with the most medals of any country, NBC is struggling to win ratings gold this season as TV viewing habits and online consumption has become a game changer for the network.

NBC’s coverage of the Games is drawing audiences three times bigger than ABC, CBS and Fox combined. But while the relative ratings strength of the coverage remains formidable, the seismic shift toward online viewing has kept the Nielsen numbers from being as big as NBC hoped.

The average audience of 27.8 million viewers through the first 10 nights is down 17 percent from the 2012 Games in London.

With more competition airing live in prime time, NBC counted on the Rio Games’ ratings to be as good or better than London, the most-watched Olympics held outside of the U.S.

Through Sunday, online users had streamed 1.86 billion minutes of NBC’s Rio 2016 coverage, topping the combined number for London and Sochi, Russia. The network is making videos of every Olympic event available on its app and streaming its prime-time coverage of the Games for the first time.

NBC is also airing Olympics coverage on NBCUniversal’s cable networks NBCSN and Bravo during prime time, giving viewers an alternative to the events airing on the broadcast network. But the growth online reflects the dramatic change in viewing habits over the last four years, making even a typically surefire TV event as the Olympics vulnerable to how viewers are consuming content.

In the summer of 2012, Netflix had 25 million subscribers in the U.S. Now it’s up to 46 million. Millennials spend just over half of their TV viewing time binge-watching programs, according to a recent study by GfK Research, and spend far less time watching live. Prime-time TV usage overall among 18- to 34-year-olds has declined 25 percent since the 2012-13 season.

“The number of available viewing choices has grown exponentially over the years and with that choice comes some level of fragmentation that we know will inevitably continue to grow,” said Billie Gold, vice president and director of programming research at Amplifi US, a media buying service.

Gold believes the drop in TV viewing of the Games among viewers ages 18 to 49 — down 25 percent compared with 2012 — can also be attributed to the availability of Olympic content online both through NBC and social media.

The Rio Games are averaging a 15.4 household rating, according to Nielsen, below the figure in the high teens that NBC guaranteed to advertisers. Lazarus does not anticipate a problem meeting that commitment.