The No. 1 men's player in the United States rolled in the second set after a 7-2 advantage in a first set tiebreaker against his Spanish opponent.
Roddick goes after his third SAP crown in Sunday's title match when he faces fourth-seeded Czech Radek Stepanek, a 6-7 (5-7), 6-4, 6-1 winner against American Robby Ginepri.
"I had more chances on his serve in the first set and I thought I was the better player," Stepanek said. "When the tiebreak slipped away, I wasn't happy, but I knew it was only one set and if you want to win a match you have to win two."
Roddick won this event in 2004 and 2005, and has a 3-0 lifetime record against Stepanek, the latest in 2007 at the Legg-Mason Classic in Washington.
Sunday's winner earns $68,800.