The Spaniard bagged the first set of the semi-final 6-4 when fifth seed Raonic retired with a lower back injury.
Both players had looked weary after having completed their quarter-finals earlier on Saturday due to heavy rain and a hailstorm late on Friday.
Verdasco had resumed play one set down against seventh-seeded Anderson but turned the match around to win 6-7 6-2 6-3, while Raonic beat fourth-seeded Frenchman Gilles Simon 7-6 4-6 6-3.
"With the rain and the waiting time it is very tough because when you get up to return to the match you feel that your legs and back are stiff," Verdasco told reporters.
"But when you got out to play a final you forget all this, you have the adrenalin that comes with it," the Spaniard added.
Argentine Del Potro - the 2009 U.S Open champion - reached the final by overcoming Uruguay's Pablo Cuevas 6-2 7-6.
A former world number four, who is currently ranked 46th after missing most of last year with a wrist injury, Del Potro made use of his potent forehand throughout, much as he had done to beat top seed Robin Soderling in the previous round.
Sunday's final will be the first clay court encounter between the two players .