World number one Rafa Nadal knew powerhouse Juan Martin del Potro's biggest weakness was his shaky backhand, and the Spaniard exploited the vulnerability repeatedly at their US Open showdown on Friday to advance to Sunday's final.

The left-handed Nadal's strategy of attacking the backhand, which had the added benefit of avoiding the Argentine's thunderous right-handed forehand, did not initially pay off as Nadal dropped the first set.

But Nadal did not waiver and Del Potro began to lose control of the shot, committing 15 unforced backhand errors to just three winners from that side in the 4-6 6-0 6-3 6-2 win.

"Rafa just play even better the last three sets of the match and I couldn't hit my backhand as good as I did in the beginning of the match," Del Potro told reporters after the two-and-a-half hour contest.

"I feel better with my backhand, but it's not good enough to win a title like this or to win the top guys in a row."

Del Potro's backhand has been hindered by a series of surgeries to repair problems with his left wrist.

He knew it would be Nadal's strategy going into the match.

"He was playing me all the time to my backhand," he said.

"When you don't have that confidence to play three, four hours with a good backhand against Rafa, is just matter of time to get down your game."

Rafael Nadal of Spain after beating Juan Martin del Potro of Argentina.