Kei Nishikori of Japan returns a shot to Karen Khachanov of Russia (not pictured) on day four of the 2016 US Open tennis tournament at USTA Billie Jean King National Tennis Center in New York, US, on Sept 1, 2016. [Photo/Agencies]

Kei Nishikori battled for the second match in succession before claiming his place in the third round of the US Open on Thursday.

The sixth-seeded Japanese, the runner-up in New York in 2014, dropped his second set but came through 6-4 4-6 6-4 6-3 against 20-year-old Russian qualifier Karen Khachanov.

A rain delay of around three hours, which came at 4-4 in the third set, helped Nishikori regroup and he eased away to victory on the resumption.

"Especially in the third set he was playing really well, serving well," said Nishikori, who next plays Frenchman Nicolas Mahut.

"I had some chances before the rain delay, but it wasn't easy. After the delay I was playing good tennis. I am happy to win today."