Andy Murray has warned fellow Brit Heather Watson that she must overcome the toughest test that tennis currently has to offer if she is to keep her Wimbledon campaign on track.

The 23-year-old impressed in victories over Caroline Garcia and Daniela Hantuchova, but she now faces the world number one in the third round on Friday afternoon.

Watson insists that she will not be overawed by the occasion, which takes prime billing on the fifth day of play at SW19, yet Murray believes that all-conquering Williams is currently in a league of her own.

"Probably the biggest test in tennis right now, I would think," he is quoted as saying by ESPN when asked about the task facing his compatriot. "I saw a little bit of Serena's match yesterday. She looked extremely motivated from the beginning.

"In some of the matches at the French Open, it didn't appear that way. I don't know if that was nerves or anything. But she looked pretty fired up the other day. So it's going to be an extremely difficult match for Heather. She's going to have the crowd with her [Watson]; she has to try to use that to her advantage, try not to be intimidated.

"Just go out there and give it a go. I'm not going to go into tactics or anything, but she needs to really go for it, try not to let Serena dictate, and hopefully she can serve well. That will make a big difference."

Williams is seeking a sixth Wimbledon title this year, while also aiming to win a second calendar Grand Slam of her career.