By Alastair Himmer

TOKYO (Reuters) - Former world number one Ana Ivanovic insisted she still had the game to challenge for grand slam titles after booking a spot in the second round of the Pan Pacific Open on Monday.

The Serb, seeded 11th in Tokyo, beat Andrea Hlavackova 6-3 2-0 to advance after the Czech retired with a thigh injury after just 44 minutes.

"That's the only reason I'm still here," Ivanovic, who climbed to the top of the women's rankings after winning the 2008 French Open, told Reuters.

"I already achieved two of my biggest dreams in winning a grand slam and being number one in the world.

"But I believe I can still be at the top level and winning grand slams. Otherwise I wouldn't be here," added the 24-year-old winner of 11 WTA Tour titles.

Ivanovic, who has won more than $9.6 million career prize money, admitted she still had a little work to do to seriously challenge at the major tournaments.

"I need to work on my strength in the off-season, get a little stronger," said Serbia's tennis pin-up, who reached the quarter-finals of the U.S. Open. earlier this month.

"But it's also confidence. Beating the top players can really boost your confidence and make you start to believe."

Ivanovic, beaten by eventual champion Serena Williams in New York, will face Poland's Urszula Radwanska in the second round in Tokyo after the Polish player's 6-2 6-4 win over Japan's Kurumi Nara.

Russia's Nadia Petrova overpowered China's Peng Shuai 6-1 6-4, knocking out one of three Chinese players appearing despite their country's fierce political row with Japan over disputed islands.

In other first-round matches, qualifier Pauline Parmentier beat fellow Frenchwoman Alize Cornet 6-3 6-4.

American Jamie Hampton was another qualifier to advance, ousting France's Caroline Garcia 6-2 6-2.

(Editing by Amlan Chakraborty)