Murray is trying to win his third ATP Tour title in Memphis

The 19-year-old British number one came through 6-3 6-2 to set up a re-match of last week's semi-final in San Jose against Andy Roddick.

Roddick beat Chinese Tapei's Yen-Hsun Lu 6-4, 7-6 (9/7) in his quarter-final.

Murray raced into a 3-0 lead on his way to winning the first set against Koubek and broke twice in the second.

He did have to survive two break points in game four of the second set but, having done so, broke again in the following game.

That's two matches in a row where I haven't dropped my serve and that's a good run to be on. Andy Murray

It was the first time the Scot had played the world number 73 and he remains on course to win his second title in a week following his victory in San Jose.

"That's the best match I have played so far," said Murray.

"I hit the ball really well from the ground, didn't make too many unforced errors and took my chances when I had them.

"That's two matches in a row where I haven't dropped my serve and that's a good run to be on."

606 DEBATE: Your reaction to Murray's victory

Top seed Roddick, meanwhile, admits he'll have to up his game to gain revenge over Murray as he reached the semi-finals for the fourth time.

He said: "Andy's a tough guy and I made way too many errors last week (in San Jose) to really even deserve to be close.

"I definitely have to tighten it up a notch and not give away freebies."

Saturday's other semi-final was decided after Tommy Haas beat Sam Querrey 6-2 6-2, while Mardy Fish led Teimuraz Gabashvili 6-2 4-1 when the Russian retired with a viral illness.

In the doubles, Jamie Murray - Andy's brother - also reached the semi-finals with American partner Eric Butorac with a 6-4 6-4 victory over Israeli pair Jonathan Erlich and Andy Ram.