I don't believe in luck when it comes to sport. I think he is just mentally tougher than most other players right now and that's why he has an excellent tie break record this season.
Of course there is a lot of luck involved. Mental toughness plays a huge part, but for every shot you hit on the court, there is a chance you will miss. True, the best players are more likely to make their shots when push comes to shove, and the lower ranked players will miss more often in tough situations. Still Nadal has DFed on set points, and Sharkko has hit winners on MP.