Serena Williams looking for answers against Li Na

World number one Serena Williams has admitted that she is going to have to be on top form if she is to advance to the final of the US Open on Friday.

The American meets Chinese fifth seed Li Na, after she dispatched Russian 24th seed Ekaterina Makarova with a 6-4 6-7 6-2 win on Tuesday.

However, Williams, who required just 52 minutes to smash Carla Suarez Navarro 6-0 6-0 on the same day at Flushing Meadows, is confident of giving her opponent a tough match.

"She moves really well," Williams told reporters. "I think the challenge of playing someone like her is how do you beat a player that does everything so well?

"I'm going to have to think about that and come up with an answer."

Williams has won the last seven meetings between the two players.