The Swede had 20 aces and Roddick finished with 22 in a match that had no breaks.
The American is trying to qualify for next month's Masters Cup in Shanghai. Three of the eight berths are open.
Soderling next will face fourth-seeded Gilles Simon of France, who ousted Josselin Ouanna 6-3, 7-5.
With the win, Simon climbed into the top eight of the Champions Race, which determines qualifiers for the Masters Cup. Simon has won three titles this year and beat Rafael Nadal last week to reach the Madrid final.