The Austrian had dug deep to overcome Taylor Fritz in his previous round matchup, but faced no such difficulty today.
With Monfils offering no real threat, the 5th seed was able to outhit his French opponent and win the match 6-2 6-4 6-4 in an hour and 50 minutes.
Thiem was in fine form, hitting winners at will. The former French Open finalist did not even face a break point, but capitalized on four break point opportunities.
After such a comfortable win, the Austrian next faces a daunting challenge in the winner between top seed Rafael Nadal and Australia's own Nick Kyrgios in what is sure to be a blockbuster encounter.
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