How Petrov defended Alonso reminded me how Kimi defended attack from Fisichella in Spa 2009.
Everytime Fisichella get close to Kimi's slip stream, Kimi just hit KERS and and pull away just enough not letting Fisichella pass him after the straights.
The best overtaking opportunities and less risk location is the chicane after the long straight. Where Alonso lose out from Renault's F-Duct. That's why FA got stuck behind for 40 laps. Its not about FA's skill, its just the different nature of the Renault and Ferrari.
Another example will be Vettel stuck behind Alonso in Hungary. Overall RB6 is faster than F10 on the track. However Vettel couldn't get close enough to FA on the last two corners that feeds into the pit straight. Plus the pit straight is not long enough for Vettel to catch the slip stream as well.