The purpose of the study was to specify the changes in blood glucose concentrations in the course of repeated tournament and practice matches, and to quantify the Incidence of hypoglycaemia in elite tennis players. The study consisted of two parts. In the first, 147 tournament players completed a questionnaire about the incidence of hypoglycaemic symptoms during repeated tennis matches. In the second part of the study, the players participated in two subsequent matches (one singles match followed by a doubles) under (T) tournament (n = 57) and (P) practice (n = 20) conditions. Of the 147 players consulted, 94 (63.9%) reported experiences with hypoglycaemic symptoms during a tennis tournament (n = 80) and/or tennis practice (n = 62). The warm-up period for the second match day was identified as the most sensitive point for the occurrence of hypoglycaemic symptoms (n = 29), compared to the final stages of the first (n = 11) or second match (n = 7). Under both practice and tournament conditions, a significant (p < 0.01) drop in blood glucose concentration was found during the warm-up period for the second match per day (T: from 5.8 +/- 1.4 mmol x L(-1) to 4.3 +/- 0.8 mmol x L(-1) and P: from 5.4 +/- 1.1 mmol x L(-1) to 4.1 +/- 1.5 mmol x L(-1)). In conclusion, precautions should be taken to prevent a sudden drop in blood glucose concentration and hypoglycaemic symptoms during the early stages of a player's second tennis match in one day.