It would be a tough fight.
Russia and China are both developing next-generation fifth-generation fighters as they struggle to challenge American dominance of the international system. However, the two great powers are taking somewhat differing approaches to developing these new next generation machines. Those differences are driven by a number of factors including threat perceptions and requirements as well as access to technology and financial resources.
In terms of overall kinematic performance, the Su-57 is likely a superior performer compared to the Chinese J-20. With its three-dimensional thrust-vectoring capability and ample thrust, the Su-57 is likely to have excellent low speed high angle of attack maneuverability even with the current Saturn AL-41F1 afterburning turbofans, which are rated at 32,500lbs thrust each. The Russian jet should also have very good supersonic performance—with some degree of supersonic cruise capability even with the current AL-41F1 engines. However, once the Su-57 receives its second stage Saturn izdeliye 30 engines, which are expected to deliver roughly 28,000lbs of dry thrust and 42,000lbs of afterburning thrust, the PAK-FA should be able to achieve kinematic performance—including supersonic cruise and maneuverability—roughly on par with the Lockheed Martin F-22 Raptor. Indeed, as one now retired military official with extensive fifth-generation fighter experience had told me sometime ago: “Performance-wise it certainly looks to compete with the Raptor.”
Recommended: How China Plans to Win a War Against the U.S. Navy.
Recommended: How the Air Force Would Destroy North Korea.