Sinclair, who has been called up to the England under-21 set-up, ran at ease down the left into the box before cutting back at the by-line for Joe Allen who was denied by the feet of Steele from close range.
The second seed took the lead for the first time in the third game of the opening set when he reached a well-placed volley by Roddick and fired a down the line winner back past his unbelieving opponent.