Liverpool extended their lead at the top of the Premier League to 16 points with a game in hand as they battled to a hard-fought win at Tottenham.Jurgen Klopp's men became the first team in Europe's top-five leagues to win 20 of their first 21 games as they fought off a second-half fightback from Spurs to win courtesy of Roberto Firmino's first-half goal.The Brazil forward turned Spurs' teenage debutant Japhet Tanganga and then beat Paulo Gazzaniga with a sweet strike to give the visitors a deserved lead.However Liverpool were then grateful to poor finishing from Jose Mourinho's side - without the injured Harry Kane - to record another victory on their seemingly relentless march to a first title in 30 years.Heung-Min Son and substitute Giovani lo Celso missed glorious second-half chances to give Spurs some reward for a performance that improved as the game went on.But Liverpool, their position strengthened further by Leicester City's shock home loss to Southampton, held on to increase the sense of formality about the destination of this season's Premier League trophy.