“En” reprend “Et dégoûte Wawrinka”. On pourrait dire aussi “Et amuse Wawrinka au point qu’il en rigole de toutes des dents”.
C’est une manière de rappeler son dégoût et le rapport de causalité entre son dégoût et l’action de casser sa raquette. Il est facultatif mais alors la phrase perd de son mordant:

Et dégoûte Wawrinka au point qu’il casse de rage sa raquette.

Two questions will be adressed here, in relation with the submitted sentence : EN (Pronoun) and Celui-ci/Ce dernier (Subject identification)

Very often in French “En” is a neutral pronoun that replaces an indefinite element (i.e. not introduced by one of the definite articles Le-la-les ; or persons).

It is an extremely polyvalent word one must master since it is used all the time in day to day French, in written and spoken speech. For non natives, the pronoun “EN” is usually tricky because it has no direct, steady equivalent in their own languages.

It namely replaces elements introduced by “DE”, whether it expresses an indefinite (Tu vois DES animaux ? – Je n’EN vois pas) or a “partitif”, uncountable (Il mange DU pain – Il EN mange), or the origin (Je viens DE Londres – J’EN viens); substantives introduced by numbers – (Cet immeuble contient 10 appartements – Il en contient 10), and other cases always introduced by “DE”, whathever the grammatical function (which is quite remarkable).

The present case is more complex : EN replaces the whole CAUSAL sentence “Et dégoûte Wawrinka” .

To understand with the “DE” explanation, let us rephrase :

-“DE rage, il EN casse sa raquette = Il casse sa raquette [parce qu’il est en rage et parce qu’il a été dégoûté= EN]”.

“DE” can introduce a causality.

CELUI-CI – CE DERNIER (+ their feminine and plural forms)

The sentence contains a frequent error :

• “Et dégoûte Wawrinka au point qu’il en casse de rage sa raquette”.

Who is IL ?
It should be (in written language):

“Et dégoûte Wawrinka au point que [CELUI-CI/ CE DERNIER] en casse sa raquette de rage”.

In French, we always suppose that the main suject pronoun is the same as the previous one.

Celui-ci or ce dernier permit to indicate we are now talking about another person, quoted after the main subject :

-“Et il [Nadal] dégoûte Wawrinka au point que CELUI-CI [Wawrinka – not Nadal] en casse de rage sa raquette “.
(I watched Roland Garros the other day, so I know : it was Nadal… :).

Whereas the sentence is actually understood :

-Et [il -Nadal] dégoûte Wawrinka au point qu’il [?Nadal?] en casse de rage sa raquette.
Which leads to the confusion [WHO breaks its racket ? N or W ?].

EXPLANATION :
For this case of subject identification, the synonyms “CELUI-CI and CE DERNIER” are mostly used in written language, where it is typically considered faulty not to use them, and in formal speech ; but they are practically non-existent in spoken normal/ familiar language.

The rule is far from being respected in spoken speech thus, because the general meaning, the logic, the common sense, the tone of voice of the speaker, the gender and number of the (french) pronoun, … normally help understand who is the subject; but also because they are heavy, dusty, formal words, very useful to distinguish the obligations of the parts in a written legal agreement, but not so much to tell a family joke.

Practically, to solve the question and make oneself understand in spoken language, a simple or even a double repetition [name + subject-pronoun] will generally be made as follows:

-“et dégoûte Wawrinka au point que Wawrinka en casse sa raquette de rage” or,
-“et dégoûte Wawrinka au point que, Wawrinka, IL en casse sa raquette de rage”.

CONCLUSION – Writing French language as you speak it will be considered poor education. Repetition are banned in proper written speech. Therefore always use CELUI-CI or CE DERNIER when you write, but not when you speak … unless you address the Académie Française, auquel cas CETTE DERNIÈRE vous félicitera.