But on Friday, she had 30 unforced errors to seven winners.
Li is back in the fourth round at the U.S. Open for the first time since 2009. She had 11 aces Friday including one on a second serve on match point and won all nine points when she went to the net.
Agnieszka Radwanska needed nearly two hours to slip past No. 32-seeded Anastasia Pavlyuchenkova 6-4, 7-6 (1). The third-seeded Pole faced 10 break points, but Pavlyuchenkova was able to convert only two, while Radwanska was 3 for 5.
Serena Williams will be back on court later Friday a day after playing singles and doubles, facing Yaroslava Shvedova in the nightcap at Arthur Ashe Stadium.
Former champs Novak Djokovic and Andy Murray also have matches, while two other U.S. Open winners, Juan Martin del Potro and Lleyton Hewitt, play each other.