We can use Subsystem element to show breakdown of systems in UML. And, we can group Components into Subsystems.
But, is there a element to represent a system?
System here means the superset of Subsystems.
A system is just a component, that is composed of subsystems. And in a system of systems it will itself be a subsystem. So, each component is always at the same time a system and a subsystem - it just depends on the viewpoint of the modeler. Therefore the same diagrams can be used to show the decomposition of systems and subsystems.
In order to show, which of the components is the system of interest from your viewpoint, you can create a «system» stereotype, as suggested by @Christophe. Strangely enough, there is no standard stereotype for this, not even in SysML.