The players were handed the limb after their second-round Wimbledon matches, complete with a blue trainer on the end.
Amid the usual fans clamouring for a signature on Centre Court, the strange request stood out.
World number one Murray posted a photo of the incident on his Instagram, with the caption: “Signed a few things over the years ... this was definitely a first...”
Spanish player Nadal said it was probably not the “weirdest” thing he had ever had to sign.
When asked if the leg was removed specially for the star to sign, the 10-time French Open champion said: “Was already off. I don’t know if he took it off at that moment or not.”
Murray signed the leg after a 6-3 6-2 6-2 victory over Germany’s Dustin Brown at SW19, with Nadal signing it a few hours later after a 6-4 6-2 7-5 win over American Donald Young.
It came the day after a man snatched a towel from a boy’s hands when American player Jack Sock threw it into the crowd.
Judy Murray said the man should be “ashamed” of himself.
The young fan was later found, with Sock saying he would send him another towel.