BEIJING — As he prepares to leave Beijing, the U.S. envoy to China feels relations between the two rival powers are the most stable they’ve been in recent years. But there’s no guarantee they will stay that way.

It “remains a very challenging, often very contentious and in a long-term way deeply competitive relationship,” Ambassador Nicholas Burns told NBC News in an exclusive interview this week. “And you can’t get around that. We’re rivals for global power.”

Still, the…