Updated @ 12:15 p.m. with more info about the second round of tickets.

The sport is electronic, but the fans want to see the action in person.

Tickets for The International 2015 went on sale today, and the first round sold out in around 5 minutes. Valve has announced it will sell a second round of tickets starting tonight at 10 p.m. Pacific. The event, which takes place Aug. 3 through Aug. 8 in the KeyArena in Seattle, brings together the top teams from around the world to compete in publisher Valve’s multiplayer online arena battler. This is the fifth annual championship for Dota 2, and it already seems on pace to grow larger than The International 4. E-sports like Dota 2 and League of Legends are massively popular, and the competitive side alone — through ticket sales, merchandise, and advertising — is expected to generate around $465 million by 2017.

The International’s ticket sales are more evidence of the growth of e-sports. KeyArena can seat around 17,000 people, and it’s clear that more than that want to show up and cheer on their favorite pro players in person.

But with tickets sold out, most fans will have to watch online. They’ll get to join the more than 20 million other people who are likely going to do the same. The International 2014 had that many viewers with a peak concurrent audience of over 2 million. People watched on livestreaming site Twitch, through the Dota 2 client, and even on ESPN3.com.

With growing attendance and online viewer numbers, e-sports continues to build a massive audience that rivals traditional sports. Dota 2’s and League of Legends’ championship events easily rival the TV numbers for the NCAA basketball March Madness tournament or Major League Baseball’s World Series.

Perhaps more telling than the viewership numbers is that participation in high schools athletics is down for every sport except for lacrosse. Coaches at schools across the country blame video games — although it’s likely more complex than that and has something to do with research about the long-term effects of concussions.

So while fewer young people are getting into playing football and basketball, more and more are growing fanatical about Dota 2. And even if people like ESPN president John Skipper say that video games are “not a sport,” he’s going to have to soon deal with the fact that the generation growing up right now doesn’t agree with him.