The Dutch winger, who came on 15 minutes from time, scored with a fine strike two minutes into added time.
The draw means the Provence side have edged a point closer to leaders Olympique Lyon after the seven times French champions lost 1-0 at Paris St Germain on Saturday.
Marseille, tipped as Lyon's main challengers at the start of the season, are third with 27 points from 15 matches, six off the pace. Nice, 1-0 winners at St Etienne on Saturday, are second in the standings five points behind Lyon.
"We were another side in the second half. We were more offensive after the break and we played with three strikers," Zenden told French TV channel Canal Plus.
Lille took the lead at the Stade Velodrome after 11 minutes when defender Adel Rami headed in Michel Bastos's corner.
Bastos then poked the ball into an empty net to make it 2-0 eight minutes before the break after a misunderstanding between Marseille keeper Steve Mandanda and defender Ronald Zubar
FRESH LEGS
Marseille coach Eric Gerets added fresh legs at halftime with Ivory Coast striker Bakary Kone replacing defender Charles Kabore and Nigeria's Taye Taiwo coming on for Elamine Erbate.
Forward Mamadou Samassa reduced the arrears for the home side with an impressive header from Laurent Bonnart's cross two minutes into the second half.
Marseille, who visit Anfield in the Champions League on Wednesday, avoided a second home defeat in a row when former Liverpool midfielder Zenden curled a free kick into the top corner of Gregory Malicki goal to snatch a point.
Lille drop to eighth on 24 points.
Monaco pulled further away from the drop zone after three goals in five minutes secured a 3-0 home win over Le Mans.
Monaco are now 12th with 19 points from 15 matches, nine points above third-bottom Sochaux in 18th.
Argentine Alejandro Alonso scored three minutes into the second half after being set up by fellow midfielder Chu Young Park.
Le Mans defender Paulo Andre then deflected a Camel Meriem free kick into his own goal in the 50th minute, with striker Alexandre Licata grabbing the third goal two minutes later when he connected with Meriem's cross inside the area.
Elsewhere, Youssouf Hadji's goal 18 minutes from time earned Nancy a 1-0 win at troubled Nantes, who are 17th on 12 points.
Nancy are 15th with 19 points.
