Sebastien Haller marked his triumphant return from testicular cancer with a goal as Borussia Dortmund trounced Freiburg 5-1, while Union Berlin topped the table for 24 hours with a 2-1 victory over Mainz.
Kevin Behrens and Jordan Siebatcheu were on target as Union dethroned 10-time reigning champions Bayern Munich who visit Wolfsburg on Sunday in search of their first league win of the year after three 1-1 draws.
Haller, who had missed the first part of the season after undergoing two operations and chemotherapy, headed Dortmund's third after earlier strikes from Kevin Schlotterbeck and Karim Adeyemi against 10-man Freiburg. Julian Brandt and Giovanni Reyna also scored.
"It means a lot to me. I have been waiting for this since the first moment. I am just happy," Haller told Sky TV.
"As a striker, it is the best feeling. You are flying on a cloud. I felt like the whole stadium was on fire. It means a lot; it gives a big boost."
Coach Edin Terzic said: "It means a lot to him and us. We hope it was the first of many more goals."
Dortmund is two points behind Union and level with Bayern in a tight title race. RB Leipzig is another point back after a 0-0 in Cologne, their 18th game in all competitions without defeat.
Europa League winners Eintracht Frankfurt are another point behind from a 3-0 over ailing Hertha Berlin with the help of Randal Kolo Muani's brace.
Dortmund captain Marco Reus returned to the starting 11 for the first time since September against Freiburg, whose only win in Dortmund dates back to 2001 and who lost defender Kiliann Sildillia on a second bookable offense after 17 minutes.
Dortmund opened the scoring less than 10 minutes later when Germany defender Nico Schlotterbeck found the net against his former club from a tight left angle.
Freiburg leveled out of the blue on the stroke of half-time from Lucas Holer after poor Dortmund defending.
But their hopes were brief as Adeyemi restored Dortmund's lead in the 48th, before summer signing Haller had his emotional moment three minutes later.
Brandt scored in off the post with 21 minutes left, and Reyna in the 82nd added to the woes of Freiburg, whose frustrated coach Christian Streich was dismissed in-between.
Dortmund have won all matches since the restart, and so have Union, who top the table for the first time since early November.
Behrens converted Paul Seguin's cross from the right in the 32nd, Mainz leveled in the 78th from Marcus Ingvartsen's penalty as Seguin was adjudged to have handled the ball, but substitute Siebatcheu clinched victory in the 84th.
"We are enjoying the moment. But there is still a long way to go. Several good teams have the ambition to win the title," Union captain Christopher Trimmel said.
Lowly, Bochum beat Hoffenheim 5-2, leaving the visitors winless in 10 games in all competitions and in relegation trouble. Borussia Monchengladbach were held 0-0 by bottom side Schalke.
Augsburg edged Bayer Leverkusen 1-0 on Friday, while Stuttgart played Werder Bremen in the other Sunday game.