Japan's Aya Terakawa clocked the world's second fastest time of the season to win the women's 100 meters backstroke at the Japan Open swimming championships, laying down a marker ahead of the Olympics.

The world 50m silver medalist, who set a new Japanese record at the national championships in April to qualify for the Olympics, improved on it with a time of 59.08 seconds on Friday, beating Shiho Sakai into second place.

"I'm happy, because I hadn't expected to swim a personal best," said Terakawa, who was 0.09 seconds faster than the current world record pace at the 50m mark in 28.62.

"It was really tough at the last moment. I'm determined to swim 58.00s at the Olympics. You are not sure of the color of the medal if you swim 58.90. I'll try to aim at clocking a better time," she added.

Japanese team head coach Norimasa Hirai said: "She showed she has the right attitude needed for the Olympics. She can now compete against Anastasia Zueva of Russia and Gao Chang of China."