The 23-time Grand Slam champion, 37, won 6-3 6-3 to set up a tie with Russia's Ekaterina Alexandrova.

"I'm loving going out there," said American three-time winner Williams.

Elsewhere, Japan's Naomi Osaka moved a step closer to regaining her world number one ranking after her opponent Tatjana Maria retired at 6-2 down.

Ashleigh Barty's second-round defeat saw her surrender her eight-week reign as world number one, with Karolina Pliskova also in the running to take over at the top.

Czech third seed Pliskova beat American Alison Riske 6-4 6-7 (4-7) 6-2 to set up a meeting with Anett Kontaveit in the next round.

Elsewhere, defending champion and fourth seed Simona Halep beat American Jennifer Brady 4-6 7-5 7-6 (7-5) while sixth seed Elina Svitolina - winner in 2017 - defeated Katerina Siniakova 6-3 3-6 6-3.

Belinda Bencic - champion in 2015 - beat Julia Goerges in straight sets but there were defeats for Victoria Azarenka and Caroline Wozniacki.

Williams dominates after steady start

Eighth seed Williams - playing in her first match since losing the Wimbledon final - last won the title in Montreal in 2013 and is using this year's tournament as a warm-up for the US Open, which starts on 26 August, where she will go for a record-equalling 24th Grand Slam title.

After a slow start, in which Mertens went an early break up, world number 10 Williams fought back to win five consecutive games before serving out the opening set.

In the second set, 23-year-old Mertens broke Williams' serve at the first opportunity but Williams quickly responded and went a double break up.

She sealed the match on break point with world number 20 Mertens having made eight double faults and 19 unforced errors in the match.

"I feel like my movement is great," said Williams. "[I've] been working on my fitness, so I felt like it really was able to shine through."

Her third-round meeting with Alexandrova - who beat China's Zhang Shuai 6-4 6-3 - will be her first against the Russian qualifier.