Paris, France (Sports Network) - Former world No. 1 Maria Sharapova was a hard-fought fourth-round winner, while defending champion Li Na was stunned by a qualifier Monday at the French Open.

The second-seeded Sharapova was tested in a 6-4, 6-7 (5-7), 6-2 victory over game unseeded Czech veteran Klara Zakopalova on Court Chatrier at Stade Roland Garros. Sharapova converted on her first match point when the 30-year-old Zakopalova double faulted to end a 3-hour, 11-minute match that featured 21 service breaks.

Sharapova advanced despite piling up 12 double faults on a windy, rainy afternoon and had her serve broken three times while serving for the match on Day 9. Her power game was diffused on the red clay amid the heavy playing conditions, but she still had enough game to stave off the Zakopalova challenge and improve to 19-1 in three-set matches over the last two seasons.

Zakopalova had never advanced the second round in Paris prior to this particular French fortnight.

The 25-year-old three-time major champion Sharapova, who's now 33-5 this year, still needs a French Open title to complete a career Grand Slam. She reached semifinals here last year and in 2007.

The reigning Australian Open and Wimbledon runner-up Sharapova will meet the Kaia Kanepi-Arantxa Rus winner in the quarterfinals this week.

Meanwhile, Kazakhstan's Yaroslava Shvedova was beaten easily in the first set before roaring back to win 12-of-14 games in a 3-6, 6-2, 6-0 stunner against the seventh-seeded 30-year-old Li.

Last year, the Chinese Li reached the Aussie Open final and then captured the French Open title to become the first-ever Asian player to win a Grand Slam tennis event.

The 24-year-old Shvedova will now appear her second Roland Garros quarterfinal in three years. This is the only major tournament where she has reached the round of eight.

06/04 12:01:05 ET