Inter Milan suffered their first defeat in six games under new coach Leonardo when the Serie A champions deservedly lost 3-1 at in-form Udinese on Sunday.
Inter's Dejan Stankovic slammed in a 16th-minute opener after Thiago Motta's pass but defender Cristian Zapata soon equalised for eighth-placed Udinese with a powerful left-footed shot which Luca Castellazzi failed to stop.
Serie A top scorer Antonio Di Natale then curled a superb 25th-minute free kick into the bottom corner and Maurizio Domizzi exposed more questionable goalkeeping from stand-in Castellazzi in the second half.
Fifth-placed Inter had won five straight games in all competitions since Leonardo replaced Rafa Benitez on December 24 but, despite having a game in hand, they risk slipping nine points behind leaders AC Milan if they beat Cesena later.
The European champions were without injured trio Julio Cesar, Wesley Sneijder and Diego Milito and although they started with the same confidence they have displayed under the Brazilian, the frailty of the short Benitez era soon returned.
Leonardo's new 4-3-1-2 formation also showed its first shortcomings with a lack of width in a niggly game.
Stankovic tested Udinese goalkeeper Samir Handanovic twice and the visitors had a shout for a penalty but the hosts were constantly dangerous with Alexis Sanchez, an Inter target given their wish for an extra striker, always busy.
AS Roma beat Cagliari 3-0 on Saturday to move up to second, three points above Inter, while third-placed Napoli and Lazio in fourth can put distance between themselves and last term's treble winners with wins over Bari and Bologna later.