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Surrey won the toss and decided to put Birmingham in to bat in the first of the semi-finals.
Will Porterfield led the charge for the Bears, hitting an unbeaten 81 off 47 balls as the Warwickshire side posted a total of 194-4.
In reply, Surrey made a strong start to their innings, with Jason Roy  needing just 20 balls to reach his half-century.
After Roy's departure, former England batsman Kevin Pietersen came to the crease for Surrey, but added just 13 runs in a 16-ball knock.
Steven Davies and Gary Wilson both followed Pietersen back to the pavillion in quick succession as the required run rate for Surrey headed towards 14 going into the final five overs, leaving them too far behind their opponents to make a late charge.
Birmingham will face either Hampshire or Lancashire in the final this evening.