Injured Nishikori out of Japan tennis

Back pain has forced top seed and world No.5 Kei Nishikori to retire from his second-round match against Portugal's Joao Sousa at the Japan Open in Tokyo.

Back pain has forced Kei Nishikori to retire from his second-round match at the Japan Open. (AAP)

A back injury forced home hero Kei Nishikori to quit less than 45 minutes into his match with Joao Sousa, handing the Portuguese victory on Wednesday at the Japan Open.

A full-house of 9000 watched in agony as the top seed was unable to perform, never looking like a threat due to his physical problems, before he pulled out at 3-4 in the first set.

The world No.5 was playing his second match in a month after losing a US Open semi-final. Florida-based Nishikori was bidding this week for a third home trophy.

Fourth seed Marin Cilic struggled to hold off a potential upset from Fernando Verdasco, managing with a break in the final game of their marathon match to post a 4-6 7-5 7-5 victory.

The Croatian will lead his nation in the Davis Cup final next month at home against Argentina.

There was a first-round loss for last weekend's Shenzhen champion Tomas Berdych, who was unable to make the quick change between events.

The Czech third seed lost 7-6 (9-7) 6-1 to 36th-ranked Gilles Muller of Luxembourg.