Playing her first match of the tournament after receiving an opening round bye and a walkover when Alisa Kleybanova withdrew from their second round meeting with a viral illness, Li displayed some hints of rust, having to fight off three set points and an early break in the second before collecting the win.
Keys, who lives in nearby Boca Raton, had her veteran opponent on the ropes several times but has yet to develop a killer instinct, allowing the experienced Li to escape time-and-time again.
"She's number two in the world for a reason. She just won Australian Open for a reason," said Keys, defending her performance. "She's a great player.
"I played well at times and she just played the bigger points really, really well.
"There is a lot to be happy with but there is also some stuff I need to work on."
After American qualifier Coco Vandeweghe had ended Saturday play in the early Sunday morning hours with a centre court upset of 16th seeded Australian Sam Stosur, 19-year-old Keys, one of the promising young US talents, took the stage also thinking upset.
Li got off to a strong start, grabbing the early break and a 3-1 lead, but the big hitting Keys answered with two breaks of her own, surging in front 5-3.
Keys had her chance to take control of the contest but could not convert any of three set points, Li wiggling free with a break and forcing the set to a tie-break which the Australian Open champion easily won 7-3.
The young American again had Li under pressure in the second, grabbing the early break to go up 2-0, but could not sustain the pressure as Li, having now hit her stride, stormed through the next five games on her way to victory.
"I think it was pretty tough match, she played well, big serve, big forehand, especially when she was 3-1 down and then come back 5-3 up and serve for the first set," said Li. "During that time I didn't think about too much.
"I say, OK try to hit the ball, try to do what you have to do and save the set point.
"I think this maybe changed the match a little bit because after that I was feeling she's dropped a little bit."
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