Combined Fragments (CF) are the new features added to UML2.0 sequence diagrams (SD). They have widely increased its expressiveness power, permitting to model complex behaviours, they can be nested to allow more sophisticated behaviours. We focus on the most popular CF of control-flow ALT, OPT, LOOP, SEQ allowing to model respectively alternative, optional, iterative and sequential behaviours. They require a meticulous processing for the generation of partial order between their events. We proposed in a previous work, a causal semantics based on partial order theory, which is suitable for deriving of all possible valid traces for sequence diagrams with CF modelling behaviours of distributed systems. In this work, to deal with nested CF, we first update the formalization of sequence diagram, then we extend this semantics.