The best defense against a surprise attack is not to be "surprise". In other words, Bruce Lee always emphasized that a martial artist must constantly be aware of his surroundings. He must be trained to be cautious and alert at all times. He should never be caught napping before an attack. In this page Lee demonstrates how to defend against a surprise attack but it can be performed by doing regular practice only.
From the Pictures 1&2, Lee walking down the street, Lee noticed some one standing at the corner. Instead of walking nearby him, he leaves enough room for himself to defend against an ambush.
From Figure 3&4 As the assailant attacks, Lee counters with a quick and powerful side kick to the forward knee.
The kick is followed through completely so that it causes the assailant to reel backward.
Lee counter attacks with multiple hooking and straight punches to the face, keeping the assailant off-balance.
(1-3) The assailant follows from the rear, 
but lee is aware of him and pretends he doesn't see him.
(4-6) Before the assailant can throw a punch, Lee counters with a side or back kick to assailant's knee, Knocking him backward. 
(7-8) Lee follows up by turning to execute a groin kick.