A home defeat to Gamba Osaka had denied Urawa the chance to seal the title last weekend, but their run-in continued to unravel against 10-man Sagan.
Yuki Abe opened the scoring from the penalty spot in the 69th minute, after Naoya Kikuchi had seen red for a clumsy challenge just inside the box.
But the visiting defenders were left looking at each other with bemusement when substitute Teruaki Kobayashi rose highest to nod home from a corner in the fourth minute of second-half injury time.
The result leaves Urawa on a run of one win in six league matches and on the verge of throwing away a golden opportunity to claim their first league title since 2006.
Gamba are the main beneficiaries of Urawa's capitulation, climbing to the summit on goal difference following a 3-1 win over Vissel Kobe.
Takashi Usami broke the deadlock with a left-footed effort from 20 yards after 39 minutes, and Patric made it 2-0 at the break when he tucked the ball under goalkeeper Kenta Tokushige from close range.
Usima then doubled his tally with a low drive four minutes into the second half, before Keijiro Ogawa headed a late consolation.
Gamba and Urawa are now locked on 62 points heading into the final weekend of the season, but Kashima Antlers - two points further back - will not have given up hope of making a late bid for top spot.
Toninho Cerezo's men are unbeaten in five after a 4-1 win at the now relegated Cerezo Osaka on Saturday and host Sagan next weekend.
At the other end of the table, Vegalta Sendai ensured their safety with a 2-1 win over the basement club Tokushima Vortis, leaving Shimizu S-Pulse and Omiya Ardija fighting to avoid the final relegation spot.
Omiya - the current occupants of a place in the bottom three - suffered a 2-1 defeat at Nagoya Grampus, while S-Pulse lost 3-1 at Kashiwa Reysol.
Elsewhere, Ventforet Kofu played out a goalless draw with Tokyo and Kawasaki Frontale drew 1-1 with 2013 champions Sanfrecce Hiroshima.
