Also on Wednesday, Manchester City beat visiting Borussia Monchengladbach 4-0 in a match postponed by a day due to torrential rain.
Borussia Dortmund flattened Legia Warsaw 6-0 in the Polish capital, but favoured Tottenham Hotspur fell 1-2 in London to Monaco.
Few expected Real Madrid to struggle against Sporting, yet the Portuguese club took a 1-0 lead through Bruno Cesar early in the second half and held the reigning champions at bay until the penultimate minute of regulation time.
As the match entered the final stretch, Real Madrid coach Zinedine Zidane introduced Alvaro Morata, Lucas Vazquez and James Rodriguez, who generated the offensive flurry that forced a foul from Sporting just outside the 18-yard box, setting up a free kick in the 89th minute.
Cristiano Ronaldo hit a perfect strike to pull Real Madrid level against the club where he began his career and Morata's header in the 95th minute ensured all three points for the hosts.
The next opponent for the 11-time Champions League winners will be Borussia Dortmund, who dismantled Legia Warszawa 6-0 in Warsaw.
Legia found themselves trailing 0-3 less than 20 minutes into the match after conceding goals scored by Mario Gotze, Sokratis Papastathopoulos and Marc Bartra.
Raphael Guerreiro, Gonzalo Castro and Pierre-Emerick Aubameyang scored in the second half to make the final score 6-0.
In Manchester, Sergio Aguero's hat-trick powered Pep Guardiola's Manchester City to a 4-0 triumph against an ineffectual Borussia Monchengladbach.
Leicester City, who stunned the world last year by winning the English Premier League, delivered a convincing 3-0 performance against Club Brugge in Bruges.
Marc Albrighton scored in the fifth minute and Riyad Mahrez contributed a brace in a match that was more lopsided than the score would indicate.
Wednesday's biggest upset came at London's famed Wembley stadium, where Monaco built a 2-0 lead on goals by Bernardo Silva and Thomas Lemar before Toby Alderweireld clawed one back for Tottenham just before half-time.
But Monaco held on to claim an important 2-1 victory.
Porto squandered an early lead at home and had to be content with a 1-1 draw against Copenhagen.
Italian champions Juventus were likewise forced to settle for a single point, managing only a 0-0 draw with Sevilla in Turin.
Lyon cruised to a 3-0 win at home over Dinamo Zagreb, getting goals from Corentin Tolisso, Jordan Ferri and Maxwel Cornet.
In Germany, Bayer Leverkusen were held 2-2 by CSKA Moscow.
