The phrase "a serious rival for" is correct and usable in written English.
It can be used when discussing competition between entities, such as businesses, athletes, or products, indicating that one is a strong competitor against another.
Example: "The new smartphone model is a serious rival for the leading brand in the market."
Alternatives: "a formidable competitor to" or "a strong contender against".
The offering here is a serious rival for the title of best reasonably priced burger in London.
The person most pleased about his defection was not Farage, who now faces a serious rival for the Ukip leadership, but Ed Miliband, the invisible Labour leader.
De Gaulle evidently sensed the emergence of Pompidou as a serious rival, for the prime minister had shown toughness and nerve during the crisis, while the president had temporarily lost his bearings.
Google's big concern is that the bounty of data Facebook has about its users, including their tastes and friend connections, will turn it into a serious rival for online advertising dollars.
But the aggressive South African company is a serious rival for HRG because it already owns Bensons for Beds and Harveys in the UK plus Conforama in France as well as discount chains in Africa and Australasia.
Within 18 months it hit the 100m-user mark, which took Facebook and Twitter around four years; in November 2013 it surpassed 300m downloads, making it a serious rival for WhatsApp, an American messaging service Facebook is paying $19 billion for.
We believe that, regarding the benefits of our proposal, CSLLS could become a serious rival of MPLS in IP networks.
Sky sees ABC News as a serious rival, openly campaigning for the public broadcaster to drop its investment in rolling news.
Bodega Chacra Barda Pinot Noir, Patagonia, Argentina 2014 (£18.95, Lea & Sandeman) Argentina hasn't quite the range of pinot producers you find in Chile, but it does have a serious rival to Errázuriz for the best pinot estate in South America.
Thomson (Thomsonfly.com) flies to Málaga from nine British airports from £135 return Average high: October 23C, November 18C Mallorca is now a serious rival to Ibiza for its gastronomic restaurants, hip bar scene and fashionable crowd.
Last month music fans downloaded 500,000 tracks, making music downloading a serious rival to CDs as a platform for distributing singles.
