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The Portugal superstar, rested for the 2-0 win over Brescia last time out, scored in an 11th consecutive Italian top-flight game, matching a feat previously achieved by Gabriel Batistuta and Fabio Quagliarella.

Maurzio Sarri's side, which was far from convincing, added a second through Aaron Ramsey before Andrea Petagna's penalty offered SPAL late, but ultimately fruitless, hope. 

The reigning champion will now watch on with interest when title rival Lazio and Inter Milan both play on Monday (AEDT). 

SPAL started brightly as Petagna tested Wojciech Szczesny inside the opening three minutes.

However, Ronaldo, who scored in the reverse fixture back in September, had a goal ruled out for offside at the other end, while the unmarked Juan Cuadrado blasted high and wide with an ugly first-time effort.

Juve began to build pressure and SPAL had to put its bodies on the line to thwart Cuadrado, with Etrit Berisha smothering the Colombian's deflected shot. 

Szczesny had a brief scare when Gabriel Strefezza's low strike threatened to nutmeg him, but the Pole recovered just in time.

After Paulo Dybala had struck the post with a curler, Ronaldo celebrated his milestone outing — his 1000th for club and country — with a measured 39th-minute finish at the end of a move instigated by Ramsey, with Cuadrado providing the assist.

Giorgio Chiellini's first starting appearance since August came to an end just before the hour mark, and the ovation for the Juve stalwart had barely finished when Ramsey doubled his side's lead.

Picked out incisively by Dybala, Ramsey kept his cool to loft a fine finish over Berisha for his first league goal since September.

SPAL stuck at its task and halved the deficit when Simone MissirolI was felled in the box by Daniele Rugani, Petagna slotting in the spot-kick for a worthy consolation before Ronaldo's late free-kick hit the crossbar.