In American English, the verb rival is usually inflected rivaled and rivaling, with one l. Outside the U.S., the more traditional double-l forms, rivalled and rivalling, are standard.
Rival is one of a class of l-ending verbs whose inflected forms have lost the second l in American English. This has happened in two waves. The first group of verbs, which includes travel, fuel, and label, permanently lost the second l in the early decades of the 20th century. The second group, which includes marshal, lost the second l more slowly, moving through many indecisive trends before the newer forms finally gained predominance in the 1980s and ’90s. With this second wave, the single-l forms now prevail by a significant margin but are not fully entrenched; some still face resistance from some Americans.
Rival is part of the second wave. Rivaled and rivalled were more or less even in American texts through much of the 20th century, but, as a Google ngram suggests, the single-l form pulled away around 1980 and has only gained since then. As of 2013, rivalled and rivalling are almost nonexistent in U.S. news stories from the past year (news stories are easier to search than other types of texts, and news publications tend to be years ahead of books as indicators of usage); in the past month, for instance, a selection of about 30 of the biggest U.S. news websites had 22 instances of rivaled against a single rivalled.
The wild ride rivaled other rapids in West Virginia, Maine and Washington state. [Newsday]
It was, prosecutors said, one of the largest heists in New York City history, rivaling the 1978 Lufthansa robbery. [New York Times]
“Merlin” has been massively popular in the U.K., where it rivaled “Doctor Who” in ratings. [Los Angeles Times]
Yesterday’s dramatic finish rivalled the late drama between Brentford and Doncaster at Griffin Park on the final day of the League One season. [Evening Standard]
Before the crash, Manulife traded for more than $40 a share and had a market capitalization rivalling the major banks. [Globe and Mail]
However, incomes on the West Coast had grown to the point where they now rivalled those in Auckland and Wellington. [New Zealand Herald]