Madison Keys has claimed that she was forced to change her approach in the 6-3 6-3 defeat to Serena Williams in the fourth round of the US Open on Sunday.

Despite matching the world number one for long spells, Keys never looked capable of causing a shock against her American rival and she admits that it was difficult to stick to her natural game.

"She put a lot of pressure on me to have to kind of almost overplay. In a sense I feel like she made very little unforced errors, making me hit a lot of winners, and served really, really well," the 20-year-old told the US Open website.

"I felt like I was going for it more than I normally would have. I felt like if I didn't hit a winner I wouldn't get to the next ball."

Serena will meet her sister Venus Williams in the last eight.