Top seed Svitolina lost the first three games against Zarina Diyas but recovered to post a 6-4 7-6(2) victory.
Williams, the second seed, was playing her first match since losing to Sloane Stephens in the semi-finals of the US Open and eased to a 6-2 6-2 victory over Risa Ozaki.
Defending champion Caroline Wozniacki was in blistering form against Eugenie Bouchard, racing to a 6-1 6-1 win in less than an hour, while fourth seed Agnieszka Radwanska beat Zhang Ling 6-1 6-2.
WTA Hong Kong
Pavlyuchenkova beats Gavrilova in Hong Kong final
15/10/2017 AT 18:46
Fifth seed Elena Vesnina was the major casualty, the Russian losing 6-3 6-4 to Luksika Kumkhum.
All singles matches at the Tianjin Open were cancelled for the second straight day because of rain.

Hong Kong Open women's results

3-Caroline Wozniacki (Denmark) beat Eugenie Bouchard (Canada) 6-1 6-1
1-Elina Svitolina (Ukraine) beat Zarina Diyas (Kazakhstan) 6-4 7-6(2)
Jennifer Brady (US) beat Misa Eguchi (Japan) 6-3 6-0
6-Anastasia Pavlyuchenkova (Russia) beat Timea Babos (Hungary) 6-4 6-4
Jacqueline Cako (US) beat Priscilla Hon (Australia) 6-3 4-6 7-6(5)
Luksika Kumkhum (Thailand) beat 5-Elena Vesnina (Russia) 6-3 6-4
2-Venus Williams (US) beat Risa Ozaki (Japan) 6-2 6-2
7-Daria Gavrilova (Australia) beat Miyu Kato (Japan) 6-1 7-6(4)
Naomi Osaka (Japan) beat Alexa Guarachi (Chile) 7-5 6-4
4-Agnieszka Radwanska (Poland) beat Zhang Ling (Hong Kong) 6-1 6-2
WTA Hong Kong
Stosur made to work hard to advance in Hong Kong
09/10/2017 AT 13:11
WTA Hong Kong
Konta withdraws from Hong Kong Open with foot injury
03/10/2017 AT 11:33