By Matthew Cronin
Roddick and the fifth seeded Berdych traded service bombs and big forehands but the Czech missed two key forehands in tiebreakers -- one at set point to lose the first 7-5, the second at 5-5 in the second set tiebreaker.
Roddick clinched the latter 7-5 with a 141-mph service winner to move to within two victories of matching his 2004, 2005 and 2008 triumphs in San Jose.
"He hits the ball so hard and flat it kind of keeps you a little off balance and a lot of times you feel like you are just hanging on," Roddick said. "He forced me to play well. I was fortunate to get through it in two."
The two Americans were joined in the semi-finals by Spanish second seed Fernando Verdasco, who punched his way past Lithuanian teenager Ricardas Berankis 6-3 7-6.
Verdasco is struggling with a sciatic nerve injury but that did not stop him from serving big and unleashing his highly-effective forehand.
The Spaniard overcame the former junior world number one in the tiebreaker 7-5 when he pounded a service winner and then watched the Lithuanian err on a forehand.
"I had never seen him play before and he served well, played close to the lines and had a good forehand and backhand," Verdasco said. "I didn't see any weaknesses."
World number 11 Verdasco said the injury, which is affecting his right hamstring, was improving slowly.
"Five days ago it was really bad but it's feeling a little better and I hope it's going to be gone soon," he said.
"I think it's the best match I played of my life," said the 96th-ranked Istomin. "Now I feel like I can play with these guys and beat them."
(Editing by Nick Mulvenney)