Portugal's Primeira Liga resumed after the coronavirus lockdown on Wednesday night but it was an unhappy return for Porto after the leaders slumped to a surprise 2-1 defeat at Famalicao.
Despite the match being played behind closed doors, several Porto supporters made the short trip to congregate outside the stadium. They were left disappointed though, as their side's title hopes were dealt a blow.
Second-placed Benfica, who sit one point behind Porto, could take top spot on Thursday when the reigning champions host Tondela.
Promoted Famalicao led the table themselves earlier in the season before a poor run of form, but moved back into fifth, two points adrift of fourth-placed Sporting Lisbon.
Fabio Martins broke the deadlock for the hosts in the 48th minute, before Porto equalised 16 minutes from time through Jesus Manuel Corona.
But Pedro Goncalves had the final say for Famalicao four minutes later as they added another famous victory to their league double over Sporting this season.
Earlier, Lucas' goal in the second half secured a 1-0 win for second-bottom Portimonense against Gil Vicente to boost their survival hopes.
The German Bundesliga restarted in mid-May, while Spain's La Liga, the English Premier League and Italy's Serie A are all scheduled to resume later this month.
However, Ligue 1 in France and the Dutch Eredivisie both ended their seasons early.
The restart in Portugal has been overshadowed by bickering between clubs.
Health authorities called for games to be staged "in the fewest number of stadiums possible". Ultimately, 16 of the 18 top-flight clubs will play at their home ground.
Maritimo are blocking the league's proposal to allow five substitutions - up from the standard three - to ease the physical strain on players.
Some clubs, including Benfica, are also upset about the decision to broadcast matches on free-to-air television.