To pursue sustainable development, it is crucial to explore and prioritize economic growth and environmental protection. Global competitiveness is central to sustainable development in the complex relationship between economic growth and environmental protection. Analyzing the data of 121 countries, both developed (37) and developing (84), from 1995 to 2019, this study examines how economic fitness impacts environmental sustainability. The findings revealed that nations with higher economic fitness (0.038; -0.054) tend to have lower emissions, which is consistent with the EKC pattern, whereas increased capital investments (b = 0.137) and low consumption of renewable energy (b = -0.049) are linked with higher CO2 emissions. Similarly, the study findings highlighted the crucial role of economic fitness and the complexities of its relationship with environmental outcomes across different country groups. Moreover, the turning point for the overall sample of countries where the EKC starts to decline occurs at $63211.89 GDP per capita for high economic fitness, and for low economic fitness, it is $66070.98. Similarly, the turning points in the relationship between GDP per capita and environmental quality vary significantly between developed and developing countries. Therefore, nations with higher economic fitness experienced environmental improvements at lower GDP per capita levels in both groups of countries. This trend is particularly pronounced in developing countries, where nations with high economic fitness have demonstrated earlier improvements. In conclusion, this study contributes significantly to the understanding of the complex interplay between economic fitness and ecological sustainability.