Federer went down to Australian Thanasi Kokkinakis, while Wozniacki fell to Monica Puig of Puerto Rico. Kokkinakis entered the match ranked No. 175, while Federer was No. 1.
World No. 1 Simona Halep slipped up, falling in three sets to No. 32 Agnieszka Radwanska.
"This is nuts," Kokkinakis said after the match, according to the ATP World Tour. "I was scheduled to play a challenger next week in St. Louis...But I think I've got a good excuse as to why I can't go now. It was an unreal win for me. I've trained with him a lot of times. Everyone looks up to him obviously, such a good role model for the sport."
"He is playing unbelievable tennis this year and his invited me to train a few times and then I took what I learned from him and then sort of kind of knew how he played a little bit and just played my game and executed.
Puig -- ranked No. 82 -- took down No. 2 Wozniacki 0-6, 6-4, 6-4. Kokkinakis beat Federer 3-6, 6-3, 7-6.
Puig battles Maria Sakkari at 1:40 p.m. Sunday in a round of 32 matchup. Kokkinakis battles Fernando Verdasco in the round of 32 at 11 a.m. Monday.
Radwanska faces Victoria Azarenka at 11 a.m. Monday in the round of 16.