Nishikori began his clay-court season at the 2015 Barcelona Open Banc Sabadell where he reached the final after beating Teymuraz Gabashvili, Santiago Giraldo, Roberto Bautista Agut and Martin Kližan. He successfully defended his title by beating Pablo Andújar in the final in two sets to win his ninth career title on the tour. At the 2015 Mutua Madrid Open, Nishikori was granted a first-round bye and proceeded by defeating David Goffin in three tough-sets in the second round. He reached the semifinals after beating Roberto Bautista Agut and David Ferrer in straight sets respectively. He then lost to Andy Murray in the semifinals, and his ranking dropped to 6th as a result of failing to defend his points from the final in 2014.