: Some statistical models suggest that the first game of a match is the hardest one in which to achieve a break of serve. Tennis serve performances at break points - ResearchGate
: A study in Frontiers in Psychology explores the "return of serve" as a qualitative framework, detailing how elite players use kinematic (body movement) and contextual information to break an opponent's serve.
on grass), second-serve effectiveness remains relatively constant at around , making it a universal vulnerability for potential breaks.