With the latest. v8.50 update, Epic Games have made it possible for Fortnite: Battle Royale players to watch Fortnite World Cup matches in-game, starting from the third week of qualifiers.
As written in the official patch notes:
"Added the ability to spectate live matches during tournaments by selecting a player on the leaderboards.
- Note: Match must have been running for 10 minutes for live replay to be available."
You can access in-game streams for tournament matches via the Compete tab. You have to find the competition you wish to spectate, select the player or duo you want to spectate from the leaderboards and you'll be put into the spectator system.
Be aware, however, that you can only watch matches with players from your own matchmaking region; for example, someone in EU won't be able to spectate a competition between players in NA West.
Of course, you can simply change your matchmaking region to view overseas competition, as many players do when finding matches.
This feature will be available starting April 27 through the entire Fortnite World Cup process, culminating in the finals event in New York City on July 26-28.