The defending champion won 6-1, 6-2 to advance.
Both are playing for their second ATP title of the year.
The American will break into the Top 40 next week.
Halep, Wozniacki to meet in Porsche Grand Prix semifinals
The Bulgarian is just 12-7 this season and has not reached a tournament final.
Djokovic leads the race to Paris, but is Nadal right where he wants to be as well?
Looking ahead to a strong draw in Stuttgart, and Rafa's important week in Barcelona.
Unlikely hero Simona Halep has endeared herself to fans with a refreshing style of play.
Nerves and Novak: Can Rafael Nadal fend off both of these obstacles?