In the wake of the intensifying Sino–Japanese rivalry in Asia, a new third-party market cooperation (TPMC) framework emerged in late 2018. The new mechanism was inaugurated around the time of former Japanese Prime Minister Shinzo Abe's visit to Beijing, displaying new attempts at Sino–Japanese economic diplomacy. Nonetheless, this article highlights the absence of major cooperative outcomes and suggests a pessimistic vision of TPMC's future, as three pairs of politico-economic gaps have led to asymmetries in the approaches adopted by China and Japan. First, a government-to-government policy asymmetry occurred between China's initiatives and Japan's responsiveness, as the latter merely utilized the TPMC as a temporary diplomatic tool to restore bilateral relations in late 2018 and appeared to have lost its policy momentum in the following years. Second, asymmetries in business modes and investment management between the two countries' enterprises have led to technical barriers in materializing the two governments' agreements into specific TPMC business collaboration. This study illustrated a project of Sino–Japanese collaboration in building a high-speed railway in Thailand, and saw this failed case as a warning for existing gaps of commercial modes. Third, both countries are now confronted with domestic underdeveloped public–private partnerships between state and private sectors. This article also aims to provide some policy-oriented implications for steadily implementing the Sino–Japanese TPMC in the future.