Hewitt won two more points than Roddick, trying to cap a week in which he started by upsetting No. 3 seed James Blake with a spot in the finals.
"I was lucky to get through," Roddick said. "He had me up against it for the better part of the match. I kind of just snuck out of the tiebreaker there, and I played my best tennis in the third set."
Roddick advanced to his second final this year and his fourth overall in Memphis where his lone win came in 2002. He will play Czech Radek Stepanek, who breezed past qualifier Dudi Sela of Israel 6-3, 6-1 to reach his second straight ATP World Tour final.
"I'd love to win this tournament again. It's been a long time," Roddick said.
Victoria Azarenka of Belarus won the Cellular South Cup championship on Saturday for her second career title, beating friend Caroline Wozniacki of Denmark 6-1, 6-3 in a match that lasted 70 minutes only because the Dane talked with her coach twice and needed a trainer to bandage the inside of her right ankle.
Azarenka, who will be ranked 11th in the world on Monday, also won in Brisbane earlier this year. She improved to 13-1 this year and 2-4 in finals.
"It feels really good because what can feel better than winning another title, especially the second one ... in the three tournaments I've played. I feel confident, and I want to keep going the same way. I think I deserve it because I worked really hard," Azarenka said.
Azarenka followed up by taking 12 of 14 points and a 4-1 lead in the second before Wozniacki called for the trainer. When Wozniacki hit a forehand long on a second championship point, Azarenka dropped her racket and covered her face with both hands before meeting her friend at the net.
"She was just playing great," Wozniacki said. "I didn't feel like I was playing badly. We had a lot of long rallies, and I felt like whatever I did was just coming even faster back."
The duo then teamed up and beat Yuliana Fedak and Michaela Krajicek in the doubles' final 6-1, 7-6 (2).
In the semifinal between former world No. 1s, Hewitt from Australia, is working his way back from major hip surgery last August and playing in his first ATP World Tour semi-final since August 2007. Roddick came in with a 15-3 record this year but was impressed with Hewitt's quick return.
"It's another testament to Lleyton's willpower, which I don't think has ever been questioned in the first place," Roddick said.
Hewitt broke Roddick in the first game and again in Game 5 while cruising through the first set in 31 minutes.
The second set took an hour as Hewitt fought off six break points and Roddick two in forcing the tiebreaker. Roddick took control by winning four of the first five points, shaking off an overruling that pulled Hewitt within 4-2 after the American passed on a shot right in front of him because he saw it as out.
Roddick won three of the next four to win the tiebreak and the set.
"I played pretty well, served well for the majority of the first set. I returned especially well in the first set and gave myself some opportunities in the second set and couldn't quite take the couple of break points I did have the opportunities. He came up with a couple of big shots at the right time," Hewitt said.
The men currently tied with 26 ATP titles apiece, third behind Roger Federer (57) and Rafael Nadal (32) among active players, cruised through the first six games of the third set before Roddick got a chance at break point following a nice back and forth at the net. He said he guessed right and broke at the last second to hit a crosscourt winner.
Hewitt then hit a forehand into the net as Roddick went up 4-3 and served out for match.
Stepanek, trying to become the ATP Tour's first three-time winner this year, needed just 57 minutes to get into the finals for the second straight week. He is off to a career-best 16-2 start that includes wins at Brisbane and San Jose.
The Czech didn't win his 16th match until May last year.
"I'm feeling great on the court," Stepanek said. "With every match, the confidence grows and grows. I'm very happy for that. I'm healthy, I'm not having problems with my body, which is very important for me, and I can build on that."
Sela was trying to become the first qualifier to reach the final in Memphis. He won two qualifying matches to play his way into the tournament this week.
"He didn't give me any chances," the Israeli said. "I think he's playing very well, and I think he's going to win the tournament. If he's playing like that, he's going to be tough to beat."