In a small bowl, whisk together ketchup, sugar, soy sauce, lemon juice and chilli. Season generously with salt and black pepper.
Line a baking tray with aluminium foil (for easy clean up later on). Rinse the chicken drumsticks and pat dry with paper towel. Place them in the tray and pour the sauce over the chicken, tossing to mix them up, and marinate the chicken for several hours or preferably overnight.
Preheat the oven to 200 degrees Celsius. Bake the chicken drumsticks in the middle rack for 30 minutes, tossing the chicken halfway through until the chicken are cooked. Turn the chicken drumsticks skin side up, transfer the chicken drumsticks into the highest rack, turn up the heat to the maximum and bake for another 5 to 10 minutes until the skin is slightly charred and crisp. Remove from the oven and serve immediately.