What a champion.
That said, I hope the knee issue isn't serious. If he can't be 100% to face Youznhy tomorrow, might as well with withdraw and focus on getting fully fit for Roland Garros. No risks should be taken, especially considering his usually fragile physique.
Juan is so unlucky, he can never be fully healthy for a long stretch, it seems. Oh well, I'm happy and relieved he got the US Open title before he had his career-threatening wrist injury. Regardless of what might happen in the future, he can retire a happy man with his Grand Slam title