Milos Raonic has admitted that he will have to improve his serving performance during his US Open round two encounter against Paul-Henri Mathieu.

World number 16 Raonic struggled to land his first serve throughout the majority of his opening-round victory against Santiago Giraldo earlier today.

The Canadian has conceded that he must raise his game if he is to progress any further at Flushing Meadows.

"I'm just happy with the outcome and that I managed to sort of make the most of that moment and just find a way to win," Raonic told reporters. "Everything else I've gotta hope gets better in the next round. I don't think I struggled with my serve that much in a long, long time.

"A lot of double faults, and it wasn't just one double fault per game. When I double faulted, I double faulted consistently a couple times in a row, so that made my job a lot more difficult and it gave him a little bit more freedom at the same time."

Raonic had never won in the main draw in New York prior to today.