In 1984, during a major NATO exercise, Royal Air Force Flight Lieutenant Mike Hale intercepted a U-2 at a height of 66,000 feet (20,000 m), where the aircraft had previously been considered safe from interception. Hale climbed to 88,000 feet (27,000 m) in his Lightning F3.
From
Wapedia - Wiki: Lockheed U-2.