LONDON • What a difference a year makes. This time last season, Chelsea began the new year in 14th position in the Premier League, having rounded off 2015 with a drab 0-0 draw with Manchester United and recorded only five wins from their first 19 games.

They enter 2017 as an unstoppable winning juggernaut, at the top of the table and having equalled Arsenal's record of 13 consecutive Premier League victories in a single season following their 4-2 dismissal of Stoke City on Saturday.

The transformation of last season's underperformers under Antonio Conte has been staggering.

Gary Cahill has been reinvented at centre-back, Cesc Fabregas was back to his string-pulling best, Eden Hazard is once again tormenting defences.

But no one exemplifies the turnaround at Stamford Bridge better than Diego Costa.

Last season, the Spaniard was a villain, picking fights and squandering his talent.

This season, he is transformed.

The ferocious aggression is still there, but it is now channelled - exemplified by his goal on Saturday, when he barged the not inconsiderable figure of Bruno Martins Indi to the turf before lashing into the top corner.

Already this season Costa has been involved in 19 goals (scoring 14 and assisting five). No one in Europe has racked up more.

So what has changed?

"Things are coming out well for me this season," he said.

"The team has changed, not just me. Last season, we felt a frustration. We weren't the team we should be. I wasn't the player I should be."

Key to Costa's resurgence has been his effort to cut the nastiness and niggle out of his game.

His raw physicality means his disciplinary record will never be saintly - he already has five bookings this season - but after collecting 10 yellow cards and a red last season, he says that he knew that he needed to change.

"I know when I do bad, so I thought about it and knew I had to improve that aspect because, here in the Premier League, the reality is there's no mercy," he said.

"A lot of the time it seemed like (the referees) were against me. There was no other way, because if they're not going to change, I had to change."

At the moment, Costa appears to be the most irreplaceable cog in Chelsea's well-oiled machine and an injury to the Spaniard is probably the rest of the league's best hope of derailing the London side's unstoppable march to the title.

As for Chelsea, they march on to tomorrow's meeting with Tottenham replete with morale, momentum and a little bit of Costa magic.

THE TIMES, LONDON