Kei Nishikori advanced to the US Open semi-finals after a gruelling five-set win over Australian Open champion Stanislas Wawrinka.

Nishikori somehow found the energy to beat third-seed Wawrinka 3-6, 7-5, 7-6 (9/7), 6-7 (7/5), 6-4 in four hours and 15 minutes.

For Nishikori, it was an especially taxing encounter and a second successive match that lasted in excess of four hours, after his marathon late-night win over Milos Raonic in the fourth round.

Wawrinka wrapped up what would be the quickest set of the match, taking the opening exchange 6-3, before Nishikori fought back to level the match with a 7-5 second-set win.

The Japanese 10th seed looked determined to do it the hard way, winning the second set in a tie-break, only for Wawrinka to level it at two sets apiece with another tie-break in the fourth.

In a nerve-wracking ending, both players held serve for nine straight games, before Nishikori broke Wawrinka when it mattered to take out the final set 6-4.

Nishikori will meet the winner of Novak Djokovic and Andy Murray’s clash, which takes place later on Thursday (AEST).