The Giants' 8-3 win over the Detroit Tigers set a record low television rating for a World Series opener.
Fox said Wednesday night's game received a 7.6 rating and 12 share, and that it was seen by 12.2 million viewers, according to fast national ratings by Nielsen Media Research.
The rating dropped 13 percent from the 8.7/14 for St. Louis' 3-2 victory over Texas in Game 1 last year, which averaged 14.2 million viewers. That broke the low set the previous year when the Giants' 11-7 win over the Rangers received an 8.9.
Fox said this year's opener had a higher rating at 9 p.m. EDT (8.8 to 8.7). But viewership fell after San Francisco took a 6-0 lead behind Pablo Sandoval, who wound up tying the Series record with three home runs.
Still, Fox was the highest-rated network in prime time and had its highest-rated prime-time program since May's season finale of "American Idol."
The rating is the percentage of television households tuned to a program, and the share is the percentage of homes watching among those with TVs on at the time.
Detroit topped markets with a 33.2/46, followed by San Francisco (32.3/58). The game earned a 7.3/11 in New York, 7.0/13 in Los Angeles and 6.9/11 in Chicago.