Top seed Wang Yihan saw off Canada's Michele Li, winning 21-8 21-16 in 41 minutes.
Wang Xin was even more impressive, ending the campaign of USA athlete Wang Rena with a 21-8 21-6 win inside 27 minutes.
Although the first round is following a round-robin format, both players go straight through to the knockout phase as their groups comprised only two people.
With China also boasting third seed Li Xuerui, the possibility of an all-Chinese final looks strong.
Wang Xin said: 'Everyone who comes to the Olympic Games comes in search of the gold medal.
'It doesn't matter if I am playing foreign players or members of my own team. We are all playing to fulfil our own dream.'
Karunaratne, the world number 48, won 21-18 21-16 in 46 minutes and Tago admitted he did not know what hit him.
It felt like I woke up from sleep and the next thing I know I had already lost.Kenichi Tago
He said: 'It felt like I woke up from sleep and the next thing I know I had already lost. I don't know what happened.'
The morning's most notable European success saw Danish Mixed Doubles fourth seeds Joachim Fischer and Christinna Pedersen come through a thriller against Poland's Robert Mateusiak and Nadiezda Zieba 21-9 14-21 21-17.