NEWCASTLE, England (REUTERS) - Manchester City's hopes of retaining their Premier League title suffered a potentially grievous blow as struggling Newcastle United earned a remarkable 2-1 comeback win over the champions at St James' Park on Tuesday.

City were far from their rampant best but a goal after 24 seconds from Sergio Aguero had looked set to ease them on their way to a victory that would have put them within a point of leaders Liverpool.

Instead, Newcastle, struggling just above the relegation zone, hit back in the most unlikely fashion after halftime, with a goal from Salomon Rondon and an 80th minute penalty by Matt Ritchie.

It meant an end to the fantastic start to the New Year for Pep Guardiola's champions, who had looked ready to win their eighth straight match in 2019 once Aguero had scored the quickest goal of the season in any Premier League match.

Newcastle's well-laid plans were immediately shredded by their perennial tormentor Aguero, who volleyed his 15th goal in 13 league games against Newcastle when set up by David Silva's brave far post header after the Spaniard lost his footing.

Aguero had a second effort ruled out after latching on to a free kick that Kevin De Bruyne was ruled to have taken without waiting for the referee's whistle - a misdemeanour that saw the Belgian booked.

But City paid for their sluggish form, despite dominating possession, with Rondon taking advantage of poor defending to equalise before Fernandinho brought down Sean Longstaff in the box to allow Ritchie to send the home fans into ecstasy.

Liverpool host Leicester City on Wednesday, with the prospect of moving seven points clear of City if they win.