JAPAN'S Kei Nishikori has upset world No.2 Andy Murray to reach the semi-final of the US Open.

The 2014 US Open runner-up held his nerve to reverse the defeat he suffered at the hands of the Briton at the Olympic Games.

The 1-6 6-4 4-6 6-1 7-5 loss was Murray's second defeat in 28 games after he took out Wimbledon and the Rio gold medal.

"It was too exciting on the court, but I tried to stay calm,” Nishikori said courtside.

"In the beginning I didn't start well, he has a great return, but I finished well. The rain delay helped me to change tactics.

"I tried to hit a few more drop-shots than usual today and it worked really well today.

"I have great memories of this place and this is a great chance for me in the semi-finals.”

Serena Williams remained on course to move past Steffi Graf and win her 23rd grand slam crown after defeating Simona Halep 6-2 4-6 6-3.

The top seed's serve was broken for the first time in the tournament before Williams lost her first set.

But the American outlasted the fifth-seeded Romanian.

Williams will play 10th-seeded Karolina Pliskova, who defeated Ana Konjuh, 6-2 6-2.

More to come