Australian Open quarter-finalist Kei Nishikori has said he was responsible for Japan's first round exit from the Davis Cup this weekend after his defeat by Croatia's big-serving giant Ivo Karlovic.

Japan's Kei Nishikori during his Davis Cup World Group match against
Ivan Dodig of Croatia on Sunday. Pic/Getty Images

"I was the one who had to put up two wins for my country," Nishikori told Japanese media on Sunday after his teammate Go Soeda bowed to Karlovic in a decisive fifth match in the Japan-Croatia tie in Hyogo, western Japan.

"Given the fact I didn't get two wins here, I definitely have more room to improve," he said, according to Kyodo news agency.

Nishikori, the highest-placed Japanese man in world rankings history, was demolished by Karlovic 6-4, 6-4, 6-3 in the opening singles match on Friday. But the Japanese ace beat Ivan Dodig 7-5, 7-6, 6-3 on Sunday.

Nishikori rose two places to 18th in the new ATP world rankings table released on Monday and Karlovic four spots to 43th.

Karlovic, 208 centimetres (6 feet 10 inches) tall, whipped both Nishikori and Soeda in straight sets without facing one breakpoint on his serve.

He also combined with Dodig to beat a separate Japanese duo in Saturday's doubles. "I'll try to become a player who can always win both of my two matches in a Davis Cup series," said Nishikori, who has upset some of the world's top players in recent months.