China's yuan slipped against the dollar while falling to a near two-year low versus its major trading partners on Friday, as U.S. President Donald Trump and Chinese leader Xi Jinping held a much-anticipated call but left key issues that have stoked tensions between the world's two largest economies unresolved. During the more than one-hour-long call, Xi told Trump to back down from trade measures that have roiled the global economy and warned him against threatening steps on Taiwan, according to a Chinese government summary.