Resurgent Pavlyuchenkova battles into French Open quarter-finals
The Russian, whose ranking has plunged to 333 after a serious knee injury, is the lowest-ranked women's Grand Slam quarter-finalist since the 2017 US Open.
She will take on Karolina Muchova for a place in the last four after the Czech saw off Russian lucky loser Elina Avanesyan 6-4, 6-3.
Pavlyuchenkova trailed Mertens by a set and a break but drew on her wealth of experience on her first appearance on Court Philippe Chatrier since losing to Barbora Krejcikova in the final two years ago.
"First time (back). I didn't even practise. Lower players don't really get a chance to practise on those courts," said Pavlyuchenkova, who is using a protected ranking in Paris.
"Obviously a very nice memory going up the stairs, and that is very special. But also, not easy, because I was playing three matches on small courts, so actually just felt a little bit strange at the start."
Pavlyuchenkova was sidelined for eight months from May last year to January by an injury she feared might end her career.
The 31-year-old had to work her way back on the second-tier ITF circuit this spring and only won a match on the main tour in Madrid in late April.
But she is now the lowest-ranked woman in history to reach the last eight at Roland Garros after outlasting Mertens in three hours and nine minutes -- the second-longest women's match at this year's tournament.
"I didn't have much left after this match," said Pavlyuchenkova.
"(It) actually felt like a lost match already, but I kept on fighting every point, believing, and here I am."
Mertens was the third seed eliminated by Pavlyuchenkova, who got the better of compatriots Liudmila Samsonova and then Anastasia Potapova in her previous two rounds.
She admitted her enforced time away from tennis had given her a new perspective after more than a decade competing at the top level.
"I think people, we (are) always like rushing and we are never satisfied. Whatever you have you probably always look around and you want to be better or this is not enough," said Pavlyuchenkova.
"You need this and that and that and always going for more. I think it's a constant race in life, and in sports especially.
"Of course I also had a burnout couple years ago before the injury, and I think now I just try to evolve and look at this from a different position and really appreciating what I have.
"That's why I'm not giving up on every point when I'm on court, because I think this is great what we have, and I appreciate that more and more. So just fighting for this."