Sentence:  This set – the set of Rosol's life – was studded with aces and menacing ground-strokes that left Nadal an impotent spectator often <WSD>muttering</WSD> to himself and at the umpire regarding a perceived misdemeanour by his opponent. 
Meaning of the word marked with <WSD> tags:  To utter words, especially complaints or angry expressions, indistinctly or with a low voice and lips partly closed; to say under one's breath.