Former world heavyweight champion Lennox Lewis has said that Anthony Joshua's quarter-final win was the best performance of the Olympic boxing tournament so far.

Great Britain super heavyweight Joshua guaranteed himself a medal at London 2012 with an impressive 15-11 victory over Zhang Zhilei of China on Monday.

Joshua scored a stunning knockdown in the second round of the contest, a display that Lewis said shows the Londoner has a good chance of getting a gold medal.

Lewis told BBC Sport: "The Chinese opponent had a lot more experience, but Joshua came out, wasn't scared of the experience and hit him.

"That's the best performance of the Games so far. He threw his right hand, he did not expect that damage that it did but he saw the results of it."

Joshua faces giant Kazakh Ivan Dychko in the semi-final on Friday, one win away from an Olympic final.