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Purpose: To investigate metabolism and exercise economy during prolonged race simulation (>4 h) in a world-class, full-distance triathlete to help guide/adjust strategies for training, nutrition, hydration, and thermoregulation. Methods: Two experimental race-simulation days, designed to mimic the demands of a full-distance triathlon, were executed by a world-class male triathlete (MD; 25 y, body weight 82 kg, V ˙ O 2 max 6.2 L·min−1, blood lactate threshold ∼410 W, and 18 km · h−1 in cycling and running) who at the time ranked second in the world. Race simulation was performed 23 and 10 days prior to competing in Challenge Roth 2023, where MD won in a new world record/best time (7:24:40 h:min:s). Both test days lasted ∼4 to 5 hours with physiologic testing every ∼45 to 60 minutes in a "stationary" setting during cycling on a direct-mount trainer (∼320 W) and treadmill running (16 km · h−1), enabling gas exchange measurements ( V ˙ O 2 and respiratory exchange ratio) and other physiologic measurements of interest (ie, core temperature and heart rate). This was combined with "real activity" as repeated loops in an open-air field setting at expected race pace in swimming, biking, and running. Results: V ˙ O 2 was maintained at ∼4.2 L·min−1, with carbohydrates being the dominant fuel for oxidation as respiratory exchange ratio values dropped from ∼1 at the start of cycling to ∼0.85 during running. Cycling economy was stable, whereas a slight impairment in running economy occurred over time. Conclusion: High aerobic energy turnover and stable exercise economy can be maintained in a world-class record-breaking triathlete for prolonged period of time (+4 h), showcasing the importance of both for success in competition. [ABSTRACT FROM AUTHOR]