The objective of this case study was to verify the utility of the slope of the mean of the differences in the sum of squares between adjacent RR intervals (RMSSD-Slope) as an individual indicator of internal training load in two elite badminton players (a man and a woman) during 10 and 13 training sessions respectively. Heart rate variability records were made for five minutes before and after training sessions to calculate RMSSD-Slope as internal load indicator, as well as the Borg scale. The Stress Score (SS) was calculated as an indicator of sympathetic activity, RMSSD of parasympathetic activity and the relationship between the two systems (Ratio S/PS). The coefficients of variation observed in the resting records indicate a high day-to-day measurement variability, which could indicate different responses to the workloads performed. Linear relationship was not found between the RMSSD-Slope and RMSSD5, SS or training variables (volume, intensity and total training load). Nevertheless, inverse correlation was observed between Borg scale and RMSSD-Slope. In conclusion, the case study results support the use of the RMSSD-Slope and the SS as indicators of the internal training load reflecting athletes' individual response to the stimulus. The use of the RMSSD-Slope could be a practical, non-invasive and easy-to-use method that can be used by physical trainers and trainers.