A 32-year-old female from Mumbai presents to the emergency department on day 5 of illness with fever, severe headache, retro-orbital pain, and myalgia. Her husband had similar symptoms 3 days ago. On examination, she has a petechial rash on the trunk and a positive tourniquet test. Her platelet count is 85,000/µL, and NS1 antigen is positive. She is haemodynamically stable with no signs of plasma leakage. What is the most likely classification of her dengue illness?