Siemerink, whose season has been rather the inverse of the Briton's with his ranking falling from No 15 in January to 105 going into the match, dealt comfortably with Rusedski's serve and ran out a 4-6 7-6 6-4 winner.
However, Rusedski was pleased enough with his game and the fact that his form is clearly returning. "It's been a positive week for me. I'm looking forward to Hannover. I'll leave tomorrow and have an extra day of practice there," he said.
In fact, it was Rusedski's serve that let him down at the vital stage of the game after his Dutch opponent had snaffled the second set in the tie-break.
"The key to this match was the three double-faults I had in the first game of the third set," Rusedski explained. "I gift-wrapped that game for him. But Jan was the better player on the day. He hit some great returns and played some good tennis.
"I'm not disappointed in the least. I played a good first set and had some chances in the second. The tie-breaker came down to a few points and he took his chances."
Rusedski started his 11th semi-final of the year confidently, even though he had lost three of six matches against the 27-year-old Dutchman. He gained an early break in the third game of the opening set, but had a few hiccups in closing it out.
Siemerink, who beat Boris Becker and Cedric Pioline earlier ion the week, saved two set-points. Rusedski double-faulted and was then passed by the Dutchman on the next point, but the second seed then secured the set with an ace.
The second set stayed with serve into a tie-break, which Siemerink took with a service winner to level at 1-1.
Then came Rusedski's gift at the start of the deciding set. Although the Briton fought back while trailing 4-5, forcing four break-points, all were saved by Siemerink, who wriggled out of trouble with another superb volley and then won when Rusedski put a backhand into the net. Siemerink will play Jonas Bjorkman in the final after the world No 4 beat the US Open champion Patrick Rafter 7-6 7-6 in the other semi-final. The Swede, playing in his third semi-final in three weeks on the ATP Tour, has now earned 66 victories, the most of any player on the Tour this year.