Stan Wawrinka will play Nishikori in the semi-finals after defeating Juan Martin del Potro.
Kei Nishikori gained revenge on Andy Murray for his semi-final loss at the Olympic Games with a five-set win in the US Open quarter-finals.
The defeat was Murray's first in 12 Grand Slam matches, and his first since losing the French Open final to Novak Djokovic. Nishikori trailed one-set to love and two-sets to one, but won the final two to win 1-6, 6-4, 4-6, 6-1, 7-5.
Nishikori's win sends him into the semi-finals of a Grand Slam for only the second time in his career. Two years ago he reached the final of the US Open, after winning his only other semi-final, against Novak Djokovic.
Stan Wawrinka awaits Nishikori in the semi-finals. The Swiss star defeated former champion Juan Martin del Potro in four sets.
Del Potro was competing in New York for the first time since 2013, but could not defeat the two-time Grand Slam champion. Wawrinka will now play in the semi-finals at Flushing Meadows for the third time in for years.
Serena Williams is two matches away from beating Steffi Graf's record of 22 Grand Slam titles. The number one seed defeated Romania's Simona Halep in three sets.
Williams dropped a set for the first time in the tournament against Halep, but never trailed in the match. She won 6-2, 4-6, 6-3 in just over two hours.
The American will meet Karolina Pliskova in the semi-finals. The Czech player has reached her first Grand Slam semi-final after a straight-sets win over Ana Konjuh.
Friday night sees the women's semi-finals take place. Williams meets Pliskova in the first match. That's followed by Angelique Kerber's match with Caroline Wozniacki.