Li, the first Chinese woman ever to make the last 16 at Flushing Meadows, raced to a 4-1 lead but Sharapova hit back strongly to wrap up victory in 78 minutes.
World number 22 Li forced Sharapova into several uncharacteristic errors early on but once the Russian found her range, the result was never in doubt.
"I started the match a little bit slow," Sharapova admitted.
"I rushed into things. I rushed into my serve and my return. After 4-1, I thought I kind of relaxed and just made her hit an extra ball.
"She was hitting some good balls. I mean, once she gets a good hit on it, she can rip the ball really hard.
"But I got a few unforced errors from her, when I got to break her, and then from then on I gained a bit more confidence, started serving better and took more time."
Sharapova won five games in a row in the second set to seal victory and set up a match against Frenchwoman Tatiana Golovin, the 27th seed.
Despite the relative ease of her win, Sharapova said she had been impressed by Li.
"She's doing better and better with every year," she said.
"I mean, every time I've played against her, we've had really tough matches. I always seem to be down a break once in a while in the first or second set. So I know that it's always going to be a tough match-up."
The Russian said she will need to improve against Golovin.
"I don't feel like I can go into every single match and play great," she said.
"But the good thing about (it), was that I won the match and I knew that I figured out a way to beat her, playing not great.
"I know it's only going to be tougher. Mentally, I know that I need to step it up."
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