BRUSSELS, Feb 6 (Reuters) - A 90th-minute strike by midfielder Dries Mertens salvaged a 2-1 win for dominant Belgium over Slovakia in an international friendly on Wednesday. An early Eden Hazard penalty was the slim reward for Belgium, who had two-thirds of the possession in the first half, in a match which with Hazard's flicks and back-heels had begun to feel like an exhibition.