Osaka downs fourth seed Mertens in Wimbledon warm-up
Four-time Grand Slam champion Osaka dominated from the start, breaking the Belgian twice in the first set and staying solid on her own serve to take it 6-2.
The second set was more closely fought, going with serve until the 10th game.
A lucky net cord and two powerful groundstrokes gave Osaka two match points and she needed only one, taking the second set 6-4.
"I think it's maybe four or five years since I last played on grass but I'm super excited to be here," Osaka said after the match.
"I don't really have much experience on grass but to be able to close it out in two is something I'm very happy about," added the 26-year-old.
Osaka, now ranked 125th in the world, has never got past the third round at Wimbledon, her four Grand Slam wins coming on hard courts in Melbourne and New York.
Her next opponent in Hertogenbosch will be either Bernarda Pera from the United States or Suzan Lamens from the Netherlands.
Osaka appeared relaxed, dancing along with the music at changeovers and appearing to share a joke with her team in the stands.
"I tried to play every point as well as I can and hopefully that will take me further and further. I guess I'm really happy... It was a really nice atmosphere," she said.
Playing in only her second Grand Slam since giving birth to her daughter last July, Osaka was within a point of dethroning Iga Swiatek in the French Open second round.
World number one Swiatek went on to win her third successive title at Roland Garros but Osaka handed her the sternest test she faced on the clay.
"My match against Iga was really amazing for me even to play. I just need to play tighter matches like that because I felt like I was playing well," she said.
