Fourth-seeded Nishikori disappointed his Japanese supporters, falling to the Spaniard 7-6(2), 5-7, 6-3 in two hours and 14 minutes.
Nishikori got off to a lively start, however, to go ahead 4-1 in the opening set, but the Spaniard battled back feverishly and forced the local star to lose the final tie break on an unforced error.
Nishikori, complaining of back pain, had to take a medical timeout in the third set and failed to find his opening composure after that.
"I've been struggling with back problems over the past four or five years and I was in pain today," Nishikori was quoted as saying by Xinhua
"It was hard for me to stay mobile and keep my position. Under the circumstances I did reasonably well, although it's the final results that really matter," he said.
Almagro, who won the last three straight games with some devastating ground strokes and adept net play to keep his opponent on the back-foot, broke Nishikori's serve for the fourth time in the final set, and Friday's win marks his 35th match win of the season.
Almagro will play top seed and 2008 runner-up Juan Martin del Potro in the semis Saturday, following the Argentinean dispatching Ukrainian Alexandr Dolgopolov 4-6, 6-4, 6-2 in just over an hour, with an impressive array of cross court shots, following a rocky start after his first service game was broken.
Third-seed Canadian Milos Raonic cruised into the semis with a convincing 6-3, 6-3 win over Lukas Lacko of Slovakia and will meet Croatian Ivan Dodig in the semifinals who beat Jarkko Nieminen of Finland 6-2, 7-6 (3).
--IANS (Posted on 04-10-2013)