Open. Entering Wimbledon with continued injury woes, expectations were low for Nishikori to make a deep run. He beat Christian Harrison and Bernard Tomic before upsetting Nick Kyrgios in the 3rd round. In the 4th round, despite facing some elbow pain, Nishikori came from a set down to beat Ernests Gulbis, reaching the quarterfinals of Wimbledon for the first time. Thus, Nishikori had succeeded in reaching the quarterfinals of every grand slam event. In the quarterfinals, he lost to eventual champion Novak Djokovic in four sets. At the US Open, Nishikori was seeded 21st. He defeated Maximilian Marterer in straight sets in the first round, facing Gaël Monfils in the second. Monfils was forced to retire in the second set after sustaining a wrist injury during the match. Nishikori then defeated 13th seed Diego Schwartzman and Philipp Kohlschreiber to reach his third quarterfinal at the US Open. There, he faced Marin Čilić in a rematch of the 2014 final. He dropped the first set 2–6, but pulled out a close five-set victory, breaking Čilić in the final game of the fifth set to win 6–4. He advanced to the semifinals, losing to Novak Djokovic in straight sets, 6–3, 6–4, 6–2. Nishikori was the top seed at the Moselle Open. He defeated Peter Gojowczyk and Nikoloz Basilashvili before being upset by qualifier Matthias Bachinger in the semifinals in three sets. He then participated in the Japan Open seeded third, defeating compatriot Yūichi Sugita in the first round, Benoît Paire in the second, fifth seed Stefanos Tsitsipas in the quarterfinals, and eighth seed Richard Gasquet in the semifinals. In the final, he was upset by unseeded Daniil Medvedev. At the Shanghai Masters, Nishikori was seeded eighth, defeating Wu Yibing and Sam Querrey to reach the quarterfinals, where he was defeated by top seed and defending champion Roger Federer. At the Vienna Open, Nishikori was seeded fifth. He defeated Frances Tiafoe, Karen Khachanov, top seed Dominic Thiem, and Mikhail Kukushkin to reach his third final of the year, where he was defeated by second seed Kevin Anderson. At the Paris Masters, Nishikori was seeded tenth. He defeated Adrian Mannarino before facing Kevin Anderson in the third round, this time defeating him in straight sets. He was then defeated by third seed Roger Federer in the quarterfinals. However, by reaching this stage of the tournament, he accrued enough points to secure a spot in