Juventus suffered their first defeat of the season in all competitions with a 3-1 Serie A loss against Lazio in Rome.
The Serie A match in the San Siro had been overshadowed by a racism storm after Italian sports daily Corriere dello Sport's controversial headline.
Juventus have a two-point lead on top of Serie A but now can be overtaken by Inter Milan, who host struggling SPAL.
Cristiano Ronaldo warned he was "back" and ready for Tuesday's Champions League meeting with old rivals Atletico Madrid in Turin.
Cristiano Ronaldo trained with his Juventus teammates but manager Maurizo Sarri said he will need to be evaluated before the Champions League clash against Atletico Madrid.
Cristiano Ronaldo looked glum as he came off and directed some words at coach Maurizio Sarri before heading to the changing rooms.
Cristiano Ronaldo failed to complete 90 minutes in a Champions League match for the first time since February 2016.
Douglas Costa scored a dramatic injury-time winner to send Juventus into the Champions League knockout stages after a 2-1 win at Lokomotiv Moscow on Wednesday.
Juventus battled to a 1-1 stalemate at lowly Lecce, but the defending champions still stay top in Serie A as nearest rivals Inter Milan drew 2-2 with Parma.
Raheem Sterling's three goals came 11 minutes apart in the second half as Pep Guardiola's Manchester City thumped Atalanta 5-1 at the Etihad Stadium to maintain their perfect record in Group C.
Serie A: Romelu Lukaku and Lautaro Martinez each bagged a brace in a seven-goal thriller to keep Inter Milan second, a point behind Juventus.
Juventus are now four points clear of second-placed Inter Milan, who travel to Sassuolo on Sunday.
Inter Milan vs Juventus: Defending Serie A champions moved one point ahead of Inter after 7 games as they target a 9th consecutive title, and first under Maurizio Sarri.
Cristiano Ronaldo blasted in the third goal in the 89th minute to extend his record to 127 strikes in the competition to add to Gonzalo Higuain and Federico Bernardeschi's earlier efforts.
Juventus have 16 points from six games, one ahead of Inter Milan, before Antonio Conte's side play at Sampdoria later on Saturday.
Serie A: Bosnian midfielder Miralem Pjanic completed the comeback, volleying in on 63 minutes to help Juventus beat Brescia Calcio.
Atletico Madrid came back from two goals down to draw 2-2 in their UEFA Champions League opener against Juventus.
The arrests were the culmination of a year-long probe centred on the alleged blackmail of ticketing officials by the "ultras", who wanted cut-price blocks of tickets they could sell to fellow fans.