John Isner laboured for almost three hours to reach the final of the Cincinnati Masters on Saturday as he beat seventh seed Juan Martin del Potro in three tight sets.

The big-serving American survived a match point in the second set to prevail 6-7 (5-7) 7-6 (11-9) 6-3.

Isner has a chance for his second hardcourt title of the year on Sunday when he will face fourth-seeded Rafael Nadal or Czech No.6 seed Tomas Berdych.

Del Potro, who had complained of trouble with his left wrist this week, double-faulted on a match point at 5-3 in the second set, dropping his serve to give Isner a chance to turn the tide and win his first match in five meetings with the Argentine.