WASHINGTON, July 28 (Reuters) - China"s new ambassador to Washington, Qin Gang, on Wednesday wished the United States victory against COVID-19 and said great potential awaited bilateral relations, striking an optimistic tone as he arrived at his new post amid deeply strained ties. Qin"s arrival comes days after high-level talks in the northern Chinese city of Tianjin between U.S. Deputy Secretary of State Wendy Sherman and senior Chinese diplomats ended with both sides signaling that the other must make concessions for ties to improve. read more Qin, 55, a vice foreign minister whose recent past portfolios have included European affairs and protocol, is replacing China"s longest serving ambassador to the United States, Cui Tiankai, 68, who last month announced his departure after eight years in Washington. "I firmly believe that the door of China-U.S. relations, which is already open, cannot and should not be closed," Qin told reporters at his residence in the U.S. capital after arriving from the airport. "The China-U.S. relationship has come to a new critical juncture, facing not only many difficulties and challenges, but also great opportunities and potential," Qin said. He said relations kept moving forward "despite twists and turns," and added that the U.S. economy was improving under President Joe Biden"s leadership. "I wish the country an early victory against the pandemic," he added. Qin, who did two stints as a Chinese Foreign Ministry spokesman between 2006 and 2014, has earned a reputation for often pointed public defenses of his country"s positions. read more Relations between Beijing and Washington deteriorated sharply under former President Donald Trump, and Biden has maintained pressure on China, stepping up sanctions on Chinese officials and vowing that the country won"t replace the United States as the world"s global leader on his watch. China"s Foreign Ministry has recently signaled there could be preconditions for the United States on which any kind of cooperation would be contingent, a stance some analysts say leaves dim prospects for improved ties. The post of the U.S. ambassador to China has been vacant since October, when Republican Terry Branstad stepped down to help with Trump"s reelection campaign. With many U.S. ambassador posts to allied countries still unfilled, Biden has yet to nominate a replacement for China, though former ambassador to NATO Nicholas Burns is considered a favorite candidate in foreign policy circles.