Novak Djokovic of Serbia celebrates after defeating Juan Martin Del Potro of Argentina in their ATP World Tennis Final semi-final singles match in London on Sunday.

London - For a set not even the granite defence of Novak Djokovic could repel the brute force of Juan Martin del Potro on Sunday, but the Serbian rode out the storm to win the opening semi-final at the ATP World Tour Finals.

Djokovic soaked up some wrist-bending blows from the inspired Argentine before turning the match in his favour to romp home to a 4-6, 6-3, 6-2 victory at the O2 Arena.

The 25-year-old, the only player in the eight-man event with a chance of landing the $1.76-million jackpot for winning all three group matches on the way to the title, will face defending champion Roger Federer or Andy Murray in Monday's final.

He struggled to live with Del Potro's first-set salvo, however, and admitted he was in a “bit of a crisis” when he went a break down in the second, but once again showed the warrior-like qualities that have seen him end a second year as number one.

Djokovic broke a faltering Del Potro's serve twice in the second and third sets and surged to victory in two hours 11 minutes after winning 11 of the last 14 games.

“I believed that I could come back,” a good-humoured Djokovic told reporters, before showing his sweet side by offering the assembled media some chocolates.

“I had a little, let's say, crisis in today's match from 4-4 in the first set to 2-2 in the second where I didn't feel so good on the court, struggling to find my momentum and my rhythm.

“From that moment on, when I got the break back, I played very flawless tennis. That makes me very happy and also confident before tomorrow's final.”

World number seven Del Potro had played down his chances of reaching the final despite beating Federer in their final Group B match on Saturday, saying “three big names and one big guy” had reached the semis.

However, the 2009 US Open champion, back to near his best after some career-disrupting injuries, showed scant regard for reputations as he traded blows with Djokovic in the first set.

Djokovic did have a chance to break the south American's serve in the second game but Del Potro swatted away the danger with hefty blow of his racket.

Djokovic looked increasingly uncomfortable as Del Potro turned up the power throughout the set.

He survived a break point in the sixth game after a terrible smash had given his opponent a chance. But a stunning forehand pass on the run and two wasteful scuffed forehands from Djokovic cost him his serve at 4-4 before Del Potro clinched the opener with a confident love service game.

Djokovic found himself under immediate pressure at the start of the second set.

He wriggled his way out of one hole, saving three break points, but two games later Del Potro broke serve again when he prevailed in the kind of rally that Djokovic thrives on.

Both men traded heavy groundstrokes before an interlude of low slices but Del Potro then ripped a huge forehand down the line which Djokovic could barely get a racket on.

A set and break to the good, Del Potro appeared to have Djokovic where he wanted but the world number one proved he was far from a spent force when he broke for the first time in the match in the following game.

The sting suddenly went from Del Potro's game and Djokovic broke his increasingly weary-looking opponent for a 5-3 lead before levelling the match.

Djokovic picked Del Potro apart in the decider, breaking in the third game with a cleverly angled forehand. He broke again for a 5-2 lead and wrapped up the win with the minimum of fuss.

“I played really two good sets today,” Del Potro said. “But in the end, he's the number one in the world so if you don't play whole match your best level, it's difficult to beat him.” - Reuters