Monitoring is a cornerstone of athlete management in high-performance sport. In tennis, intermittent match-play, frequent transitions between training and competition, and contextual variability create a complex monitoring environment. Although numerous technologies quantify physiological, mechanical, and subjective responses, their use remains inconsistent. This scoping review aimed to map the technologies and metrics used to monitor training load and recovery in high-performance tennis, and to evaluate their practical applicability across different monitoring horizons. PubMed, Scopus, and SportDiscus were systematically searched. Studies reporting tennis-specific monitoring of physiological, mechanical, or perceptual metrics during training and/or match-play were included. Studies were categorised according to monitoring horizons (session/match; training blocks: 1–16 weeks; seasonal: > 16 weeks), metric type, and application context. Forty studies were included. Internal and subjective measures – particularly session-rating of perceived exertion (sRPE) – were most commonly reported, reflecting their high feasibility and low cost. The use of external-load technologies has increased, providing tennis-specific insights into movement and stroke demands. However, heterogeneity persists in devices, variable definitions, and reported outputs. Session-level studies indicate that tennis is characterised by moderate average demands, pronounced transient peaks, and elevated psychophysiological stress during competition. The available literature may be usefully organised across three monitoring horizons: session, block, and season. sRPE appears to provide a feasible and low-burden component of monitoring, whereas external-load technologies may add value when focused on validated, tennis-specific variables and interpreted within context.