Tennis: BNP Paribas Open-Day 9
Cilic's booming serve lacked its usual accuracy and Germany's Kohlschreiber was able to control long rallies, pushing the towering Croatian around the court with pinpoint groundstrokes en route to a quick victory.
Cilic, runner-up at this year's Australian Open, only managed to get 51 percent of his first serves in play and was unable to convert any of his four break-point opportunities.
It is the third time that Kohlschreiber has reached the fourth round at the tournament in the California desert and his seventh victory over Cilic in 11 career meetings.
The win sets up an all-Argentinian fourth round battle between del Potro and countryman Leonardo Mayer, who earlier cruised to a 6-4 6-1 victory over Japan's Taro Daniel.
Del Potro is the second-highest seed remaining in the competition, behind No. 1 Roger Federer, but the Argentine said he was giving no thought to his title chances.
“I’m not thinking about that. I just want to keep winning,” he told reporters.
“It’s good for me to feel this confidence when I get to the court and I play well, but I’m going step by step. I will play another Argentinian guy tomorrow, which is a special match for both. It’s not easy when you play against a friend.”
In other matches, Feliciano Lopez beat Jack Sock 7-6(6) 4-6 6-4, Milos Raonic beat Joao Sousa 7-5 4-6 6-2, Sam Querrey took down Yuki Bhambri 6-7(4) 6-4 6-4 and Marcos Baghdatis beat Dudi Sela 7-6(5) 6-4.
(Reporting by Rory Carroll, Additional reporting by Jahmal Corner; Editing by Toby Davis/Peter Rutherford)