Li Na of China serves the ball to Angelique Kerber of Germany during their women's singles match at the Madrid Open tennis tournament in Madrid, May 9, 2012. [Photo/Agencies]

MADRID - Chinese number eight seed Li Na had to work hard on Wednesday before beating German Angelique Kerber 7-5, 6-4 in the last 16 of the Madrid Open.

Li's win over the player who had beaten her in Indian Wells earlier in the season sets up a quarterfinal tie against number one seed Victoria Azarenka.

Li needed one hour and 28 minutes to see of a tough rival in a game where her willingness to attack her rival's serve was a key factor.

She saw Kerber save a break point in the 10th game of the first set, but kept up the pressure and got the vital breakthrough in the 12th game to take the set 7-5 after 46 minutes.

Kerber then fought straight back by breaking Li's service in the first game of the second set, but Li was showing herself to be more aggressive on court and more willing to attack her rival. That was reflected in the fact that she hit more winning shots, but also in her slightly higher number of unforced errors.

Li broke back in the sixth game of the second set before forcing two match points in the tenth game. Although Kerber was able to defend the first successfully, she failed to do so at the second attempt and Li concluded a 7-5, 6-4 win.

Wednesday morning also saw women's fourth seed Aginezka Radwanska seal a comfortable 6-0, 6-1 win against Sara Errani, while Roberta Vinci had an easy victory 6-1, 6-2 against Yanina Wickmayer.

In the men's competition, Radek Stepanek narrowly lost to number five seed, David Ferrer. The eighth seed John Isner defeated Maran Cilic after two tiebreaks 7-6, 7-6, while Stanislas Wawrinka enjoyed 6-1, 6-4 triumph over Jurgen Melzer.

Angelique Kerber of Germany returns the ball to Li Na of China during their women's singles match at the Madrid Open tennis tournament in Madrid, May 9, 2012. [Photo/Agencies]