The National Weather Service issued a high-surf advisory for the area around Pillar Point in Half Moon Bay where swells were expected to top 20 feet by Thursday evening.
Organizers of the competition said in a written statement that waves up to 40 feet were possible on Friday, when a cadre of the world's leading big-wave surfers head out to the notorious Mavericks break.
Surf at the contest site was reaching 8-10 feet Thursday morning and rolling in at 8-second intervals.
Roads around Pillar Point were closed off Thursday. Spectators will be able to watch the action in video screens at a nearby resort hotel.