Both Schalke and Dortmund could be pushed down the table if third-placed Bayern Munich win at Hamburg on Saturday night to reclaim top spot.
After Dortmund went first in the table with a 2-0 win at Nuremberg on Friday night thanks to goals by Sebastian Kehl and Paraguay striker Lucas Barrios, Schalke had to come from behind to draw at home with Mainz.
The guests took a shock lead when Egypt striker Mohamed Zidan, who signed from Schalke's deadly rivals Dortmund during January's transfer window, scored after just 15 minutes on his Mainz debut.
The Royal Blues got back on level terms when Nigeria striker Chinedu Obasi, who is on loan from Hoffenheim, netted in the 59th minute.
Dortmund are now top on 43 points with Schalke second on 41 while Bayern are third on 40, level with fourth-placed Moenchengladbach, who were held to a goalless draw at Wolfsburg.
Fith-placed Leverkusen, who host holders Barcelona in the Champions League Round of 16 first-leg tie on February 14, were held to a 2-2 draw at home by mid-table Stuttgart after an 89th-minute equalizer from Austria striker Martin Harnik.
In a dramatic last few minutes both sides finished with 10 men.
Leverkusen had Czech Republic defender Michal Kadlec sent off for his second yellow midway through the second half, while Stuttgart's Cristian Molinaro was shown a straight red for hitting Andre Schuerrle in the face in the 92nd minute.
At the bottom of the table, second-from-bottom Augsburg produced a brave performance to draw 2-2 at Hoffenheim, while Hanover are seventh after their 1-0 win at Hertha Berlin.
On Sunday, bottom side Freiburg host Werder Bremen while Cologne are at Kaiserslautern.
Dortmund claimed first place in the league on Friday with an impressive win at Nuremberg in sub-zero temperatures.
Barrios came off the bench to score his first goal of the season eight minutes from time after Dortmund captain Kehl had put his team ahead just after the break.
The victory comes at a cost as midfielder Sven Bender damaged ankle ligaments and will discover on Monday how bad the injury is after an MRI scan while Kehl picked up a foot injury and is doubtful for Tuesday's German Cup quarter-final with Holstein Kiel.
