World champion Sergey Kovalev was given plenty to think about ahead of November's mega-fight with Andre Ward after being taken 12 rounds by tricky challenger Isaac Chilemba.

What was meant to be a glorious homecoming as Kovalev fought on Russian soil for the first time since 2011 turned into an awkward night as he defended his WBA 'super', IBF and WBO light-heavyweight titles.

Chilemba was regarded as little more than a warm-up for the fight with Ward in Las Vegas on November 19, but admitted pre-fight he was angry at being "disrespected" and that showed in an intelligent display which made the big-hitting champion look laboured.

The challenger from Malawi - outpointed by Liverpool's Tony Bellew in 2013 - was effective with his jab and his clever movement saw 33-year-old Kovalev repeatedly miss.

Kovalev, who went into the fight with 26 knockout wins from 30 fights, started well enough, working behind his ram-rod jab and Chilemba was happy to stay on the back foot and out of range.

But at the start of the third the challenger landed well with hooks to the body and then a right over the top as he worked his way into the fight in impressive fashion.

He also took the fourth and what had been a boisterous crowd at the start in the city of Ekaterinburg was now in silence as their hero was made to struggle.

However, the pivotal point came in the seventh when Kovalev finally found his range as a straight left rocked Chilemba before a follow-up right put him down. The challenger just beat the count and looked unsteady but was allowed to carry on by referee Charles Fitch with the bell saving him as the champion launched another attack.

Chilemba has never been stopped despite having three losses on his record and showed his toughness by not only recovering, but once again showing the tenacity to take the fight to the champion who visibly tired in the later rounds.

He was well ahead on the cards, however, and in the last a final flourish made sure of his win with the judges scoring it 117-110, 116-111 and 118-109 which did not reflect how well huge underdog Chilemba had performed.

It was far from convincing from Kovalev who struggled to find his range and he will have to improve markedly if he is to beat a genuine world-class fighter in Ward.

"The sofa experts predicted an easy fight for me, but I knew that Chilemba was a very serious contender and it wouldn't be easy with him. At first I did not see any similarities between Chilemba and Ward, but after the fight realized that there are similarities," Kovalev said.

Chilemba gave full credit to Kovalev for the win.

"I did everything to win, but unfortunately Kovalev was stronger. I'm a warrior, I will continue to fight and I think that I can still be in the mix," Chilemba said.