Lens deserved their win," said PSG coach Christophe Galtier.
Lois Openda scored one and set up another as Lens, who have only lost once in Ligue 1 this season, closed the gap on PSG at the top to four points after 17 rounds.
"We are happy to be four points behind," said Lens coach Franck Haise. "This victory proves that we can beat anyone in this championship."
PSG's last loss was more than nine months ago when they fell 3-0 at Monaco in Ligue 1, although that still left them 12 points clear at the top of the table.
On Sunday at Lens, PSG were without Messi and suspended Brazilian Neymar, but Kylian Mbappe, the Golden Boot winner in Qatar, received a rousing welcome from the Lens fans as he warmed up.
Galtier said he hoped French fans would give Messi a similar reception.
"He's going to join us on January 3, in 48 hours," said Galtier. "I hope Leo will be well received. He won the most beautiful trophy by being very good. We know he's a great and decisive player so we hope he'll be well received obviously."
"Kylian worked had," said Galtier. "He showed a very good attitude."
"Leo and 'Ney' were not there but that is no excuse."