LONDON (AFP) - Novak Djokovic insists he is willing to defy his flagging energy levels in a bid to finish the campaign with more glory at the ATP World Tour Finals.

The defending champion booked his spot in the semi-finals at London's O2 Arena by defeating Argentinian fourth seed Juan Martin del Potro 6-3, 3-6, 6-3 yesterday.

It was Djokovic's 19th straight victory dating back to his US Open final defeat by Rafael Nadal.

The Serb arrived in England only on Sunday night after winning his third straight title at the Paris Masters and fatigue has inevitably been his biggest opponent to date at the season-ending showpiece.

But Djokovic is showing once again that his appetite for titles has no limit.

"I had a great start of the season and now a great ending of the season," he said.

"I just feel good at this particular part of the year. I know most of the players are exhausted physically, mentally, emotionally, after a long season.

"But I try to find that as an opportunity for me to get extra motivated to push myself to the limit and deliver the goods when needed."