Ireland's Shane Lowry is still in the hunt for his first win of the PGA Tour season, finishing one shot off the lead on five under, but will be disappointed with his performance on the back nine, finding a single birdie on the 18th after notching four on the front nine of the course.
Meanwhile, David Skinns made five birdies and was the surprise leader of the English contingent at TPC Toronto, finishing one stroke off the lead with a 65, while Tommy Fleetwood and Matt Fitzpatrick - both three under with a 67 - were not far behind.
Americans Sahith Theegala, Eric Cole and Sam Burns, Canadian Matthew Anderson and Argentina's Emiliano Grillo each sit tied at the top with Koepka on six under after the opening round.
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"I played pretty solid," said Koepka who left LIV Golf in December of last year. "I don't think the ball striking was as good as it's been as of late, but it's tough to compare. It's been unbelievable the last six months, but it was nice to see the putter finally heat up.
"It's been nice to be back [on the PGA Tour], I've enjoyed it. I think the players have been very receptive. I haven't heard [anything negative], at least nobody has said it to my face about anything. There's been a lot of: 'We're glad you're back'.
"It's been enjoyable. I know myself, my family, everybody's happy and excited to see what this next part is."
Alex Fitzpatrick hit four birdies to finish the day on two under but a double-bogey at the second stopped the Englishman from leapfrogging his brother, while PGA Championship winner Aaron Rai struggled, making three bogeys on his way to a one-under 69.
2023 Canadian Open winner and home crowd favourite Nick Taylor made a costly bogey on the 17th but is still contending at three under.
Watch the second round of the RBC Canadian Open on Friday live on Sky Sports Golf with early coverage starting from 12:00pm. Get Sky Sports or stream with no contract on NOW.
