The Philippines trailblazer, now at a career-high of 17 in the world, beat Australia's Emerson Jones 6-4, 6-3.
Eala faces Tereza Valentova of the Czech Republic in the last 16.
The 21-year-old Eala, who this year became the first Filipina to win a WTA Tour title, was scheduled to play Alycia Parks in her opening match.
But the American pulled out at the 11th hour with injury, giving the 18-year-old lucky loser Jones the opportunity of an upset.
Eala took the late switch in her stride, and with backing from a large number of her compatriots in the crowd, got the job done in 83 minutes.
"I'm definitely not used to my opponent changing that late," said Eala, the second seed.
"It was just a new experience. I guess that's the tour, you kind of have to be ready for anything."
Eala said that Hong Kong feels like a home tournament.
"I've been competing here ever since I was very young. I've been here many, many times," she said.
"There's a huge Filipino community, and the Hong Kong community has been very hospitable towards me, my family, my team, and all of the players.
"Besides it being geographically very close to home, it feels very familiar."
Also into the last 16 was top seed Sorana Cirstea of Romania, who needed three tough sets to battle past Maria Timofeeva 7-6(6), 1-6, 7-5.
