MADRID, May 8 -- Li Na was forced to dig deep in order to defeat No. 16 seed Sloane Stephens 2-6, 6-3, 6-2 in two hours exactly and moved into the quarterfinals of the WTA Madrid Open on Thursday.

Despite her progress, Li Na was not satisfied with her performance in which she made over 40 unforced errors.

The first set saw Li Na play her worst tennis since arriving at Madrid. Perhaps unhappy at her third consecutive early start, the World No. 2 was never able to find her rhythm on court and while struggling with her own serve, she also failed to deal with that of her rival.

Stephens concentrated on not making mistakes and allowed Li Na to make a string of unforced errors.

Going into her service game at 5-1 down, Li Na had already made 16 unforced errors and although she held her serve to take the set to 5-2, it was not without two more errors and the successful defense of a set point against her.

That was to be just a temporary respite as Stephens then held her serve with ease to take the set 6-2.

The second set saw the Chinese player discover new aggression in her game and she looked to pressure Stephens' serve, forcing errors from the American and taking a vital break in the fourth game to go 3-1 ahead.

She then kept cool to hold her serve to go 4-1 up after a double fault had given her rival a break point, but then failed to close out the set as Stephens struggled in the seventh game.

The American saved two set points against her and then went 0-40 up as Li Na served for the set, before the Chinese player showed why she is ranked second in the world by taking the next six points and with them the set.

Li Na's aggression had drawn her level, but she was still committing just as many unforced errors and was also struggling with a foot problem as she had the big toe on her left foot bandaged before the decisive set.

She began the third set by against attacking Stephens, breaking her serve in the first game of the set and as her rival began to play for her earlier efforts, she held her own serve and broke Stephens again to take a decisive 3-0 lead.