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 per cent 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. ET (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."
"When you consider what they peaked at, before the blowout, it was fabulous," Fox senior executive vice-president David Hill said. "Won the night. What can you say? It was the most popular thing on American television last night."
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.