Chop watermelon into 1 1/2 " cubes. Divide the watermelon in half and put one half in the blender. Puree.
Heat oil in a large skillet or wok. Add cumin seed. After about 10 seconds, add watermelon juice. Add chile powder, turmeric, coriander, garlic paste and salt. Bring to a boil. Reduce to a simmer and cook for about 5 minutes.
Add cubed watermelon, sugar and lemon juice. Cook on low heat for 3 minutes, tossing well to coat the watermelon with the spices.