The 36th-ranked Anderson knocked off Isner, the top-seeded American, a day after upsetting Andy Roddick in the quarterfinals.
He'll face qualifier Marinko Matosevic of Australia in the final on Sunday.
Isner was broken once in the first set and then double-faulted away the first point of the second-set tiebreaker against the seventh-seeded Anderson.
Isner entered with a 9-1 record in tiebreakers this season.
"I'm really excited because I couldn't have played a better match today," said Anderson, who won his first career title at Johannesburg last year. "Fortunately for me, John gave me a double in the tiebreak and I held onto my serve all night." The 173rd-ranked Matosevic needed five match points to close out his 2-hour, 54-minute semifinal match over Israel's Dudi Sela.
"I'm very happy but I'm very tired," said the Australian, who reached his first career ATP Tour final. "I was just fighting off cramps. ... In the humidity I just sweat a lot and cramp. I cramped after my match on Wednesday, too." Matosevic had two match points on his serve at 5-3 in the third set, but he made four straight errors to lose his game. He finally won on his third match point in the third-set tiebreaker when the 75th-ranked Sela netted a backhand.
Sela had won all three previous matches between the two players.
"He was leading in the first, second and the third but I thought I played much better except for the important points," said Sela, who left the court without shaking Matosevic's hand. "It was a matter of who was more lucky in this kind of match and he was more lucky." Matosevic said that the two are good friends and he wasn't bothered that Sela left the court immediately after the match because he was sure the Israeli was upset by the loss.
