The ninth-seeded Wozniacki, who has never fared very well on the London grass, is going through a mediocre season. She also lost in the 2nd round at the French Open.
The 196th-ranked Cetkovska, who reached the fourth round at Wimbledon two years ago, erased the six break points she faced and hit 30 winners.
A qualifier from the Czech Republic, Cetkovska's 2012 was cut short after the London Olympics because of a right ankle injury. She returned this year and has nosedived in the WTA rankings.