Rees-Zammit returned to rugby after an attempt to crack the NFL last summer and has started five of Tandy's six games in charge, the exception being the South Africa fixture when he was unavailable.
The full-back has enjoyed success with Bristol - who are third in the Prem and in the last 16 of the Champions Cup - but only one last-gasp win against Japan on the international stage.
Rees-Zammit won 17 of his first 32 international games before moving to America but has returned to a different Wales team.
"No regrets," he said. "It was an amazing experience over there but I missed playing rugby and playing for my country.
"I love being back in camp and around the boys again, getting stuck back into a Six Nations.
"It's obviously been a tough couple of years for Welsh rugby, but we're trying to do everything we can to get fans off their chairs and play exciting rugby.
"Obviously, it doesn't happen overnight but we're working really hard behind the scenes to make that work. It will come."
Rees-Zammit will be hoping to cause an upset against the Scots then watch United extend their unbeaten run at Everton on Monday.

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