Former Chelsea midfielder Michael Ballack has hailed the Antonio Conte effect after a stunning first half of the season at Stamford Bridge.

Italian boss Conte has taken the side, who struggled so badly under Jose Mourinho last season, and guided them eight points clear at the top of the Premier League.

And Ballack, who racked up over 150 appearances in West London, has been impressed by the way Conte has taken the title race by the scruff of the neck in his first season in England.

"The surprise was, last year, that they were so bad,'' Ballack said. "That they are top of the league now is not a great surprise because they have the advantage of not playing European football.

"The team looks really fit, the team have adapted really well to the new coach, who has adapted really well to English football, and the team is playing to their strengths, making very few mistakes, not conceding many goals.

"That's the key if you want to win a title -- to be solid.

"At the beginning they conceded a few goals and then they changed tactics and went to 3-4-3. That was important as well, the players feel comfortable.

"If you want to perform at 100 percent you need trust at the club, with the people you work with and Conte is doing that.

"That's a lot of factors coming together and that's why Chelsea are dominating the league so far.''

Former Chelsea midfielder Michael Ballack was full of praise for new manager Antonio Conte and striker Diego Costa. Darren Walsh/Chelsea FC via Getty Images

Ballack says that trust has also extended to Diego Costa, who is back in the fold -- and scoring -- after he was left out of the team against Leicester amid speculation of a big-money offer from China.

Speaking at the launch of international tournament Star Sixes at the O2, the former Germany captain added: "First I have to congratulate Costa for getting an offer like that! I never got one.

"But this is normal. You love what you do but it's still a job and you are limited in time earning money and you think about it if someone makes you a crazy offer.

"I think it is an emotional aspect, and also with toughness that Conte took him out of the squad, which was an understandable situation.

"But also the answer's good from Costa. They are two really high-class professionals dealing with each other.

"One is the coach, the other is the player and overall there is the club.

"He has a contract and if he understands that this is the right club and he should do his job, then it should work.''