The British women???s No 1 takes on the defending champion knowing a victory would give her the highlight of her career so far.
Watson convincingly defeated former world No 5 Daniela Hantuchova in straight sets in the second round and will go into the match with confidence.
Murray, the men???s winner at SW19 in 2013, appreciates how difficult it will be for her to see off Williams, one of tennis??? all-time great players, as well.
He knows the American has momentum but sees factors Watson can use to her advantage in front of a packed Centre Court crowd.
Murray said: ???It’s probably the biggest test in tennis right now. I saw a little bit of Serena’s match on Wednesday, 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.
???It’s going to be an extremely difficult match for Heather but she’s going to have the crowd with her.
???She has to try to use that to her advantage, try not to be intimidated and just go out there and give it a go.
???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.???
Watson has never reached the last 16 before but has faith she can change that with what would be a momentous victory.
She said: ???Do I believe I can beat her? Absolutely. There’s no point even going out there if you don’t believe you can win because then there is no way I could possibly win anyway.
???I have to believe I can win from the first moment, from now. I’ll have to be very aggressive and push myself out of my comfort zone against her.???