Somdev, ranked 551, defeated 78-ranked Phau 6-3 6-2 6-3 in match that lasted nearly two hours.
The 27-year-old Indian will next face the winner of the match between Italian Simone Bolelli and Polish Jerzy Janowicz.
Somdev was in a fine form Monday, converting two of the three break points that came his way and also hit a couple of aces to win the first set in 31 minutes.
The second set lasted 44 minutes during which Somdev double faulted once but bagged two of the four break point chances.
It took another 35 miniutes for Somdev to win the third set and wrap-up the match. Somdev broke the German twice and had a better second serve winning percentage.