Sandro Tonali (L) celebrates after scoring during the Serie A football match between Lazio and AC Milan at the Stadio Olimpico (AFP)
Last champions in 2011, Milan became favourites for the Scudetto without even kicking a ball on Wednesday after Inter's incredible 2-1 defeat at Bologna left the champions two points behind their local rivals.
Few could believe their eyes when Radu, in his first Serie A start of the season in place of the injured Samir Handanovic, misjudged a throw-in under pressure from Nicola Sansone to the point that the Italy international was able to tap in the winner from inches out.
The blunder was so huge that the Gazzetta Dello Sport compared it to an error by Giuliano Sarti way back in 1967 which handed that year's title to Juve.
Radu left the Stadio Renato Dall'Ara field in tears while teammates consoled him and Denzel Dumfries tried to block pitchside TV cameras from filming the shaken keeper.
Meanwhile audible cheers went up around the 'Rosssonero' half of Milan as the momentum swung clearly in their side's favour.
Three wins and a draw from their final four fixtures will guarantee the title thanks to Olivier Giroud's brace in February's derby ensured Milan have the better head-to-head record should both teams finish on the same points.
It is a brutal turnaround for Inter who last weekend, after cruising past in-form Roma, looked every inch the champions elect right up until Sandro Tonali snatched a stoppage-time winner at Lazio which kept Milan two ahead.
Now Simone Inzaghi's side travel to Udinese who are flying in the run-in and hammered Fiorentina 4-0 not long before Radu made possibly the biggest gaffe of the campaign.
Another slip-up could finally bring a decisive turn to a hugely entertaining title race which has swung all season.
Another derby could be crucial to the outcome at the other end of the table, as Sampdoria host Genoa on Saturday with Serie A survival at stake.
Samp have been awful of late and have been dragged back into the relegation fight following three defeats and a draw which have left them five points from their local rivals the drop zone.
Closing in on safety are Salernitana, who face Atalanta on Monday on a run of three straight wins which have put them level with Genoa and three points from Cagliari.
The Sards sit just outside the bottom three and kick off the weekend's fixtures with the visit of Verona.
Player to watch: Gerard Deulofeu
Udinese have been crashing in the goals in recent weeks and in Gerard Deulofeu they have one of the league's in-form players heading into their clash with Inter.
The Spaniard has scored 12 times from the wing this season and has been key to a team which since the start of April has found the net 17 times in six matches.
10 - The number of points which will guarantee Milan their first league title in 11 years.
2 - After Napoli collapsed at Empoli last week, a once four-way title race has been shaved down to two teams.
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