DUBAI, Feb 1 (Reuters) - South Africa's Richard Sterne maintained his one-shot advantage at the halfway stage of the Dubai Desert Classic on Friday with a birdie on the 18th as world number eight Lee Westwood slipped six shots back. Five-times European Tour winner Sterne, who carded a two-under-par 70 for a halfway total of 132 (12-under), found the going a great deal tougher in the swirling winds than when he opened with 10 birdies in a flawless round on Thursday.