Initally overpowered

Like Federer, Djokovic also found himself initially overpowered by the wrist-bending forehand of the towering Del Potro but came through what he described as a "crisis" to ultimately romp to victory and stay on course for the $1.76 million jackpot for an undefeated champion.

The 25-year-old, the only player in the eight-man event to win all three round-robin matches, was a set and break down but once again showed the warrior-like qualities that have enabled him to end a second consecutive year as world number one.

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

"I believed that I could come back," a good-humored Djokovic told reporters, before softening up the assembled media by offering up boxes of 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 despite beating Federer in their final group match on Saturday, saying "three big names and one big guy" had reached the semis.

Hefty blow

However, the 2009 US Open champion, back to near his best after some career-disrupting injuries, showed scant regard for reputations with his thunderous forehand working Djokovic over.

Djokovic looked increasingly uncomfortable as Del Potro pinned him way back behind the baseline and he spent most of the first set soaking up punishment.

He survived a break point in the sixth game after a miss-hit smash gave his opponent a chance but a stunning forehand pass on the run by Del Potro and some uncharacteristic Djokovic errors handed the initiative to the South American.

Del Potro clinched the opener with a confident love service game and 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 finished off a stunning baseline exchange by ripping a monstrous forehand that brought gasps from the crowd.

Del Potro appeared to have Djokovic where he wanted but the world number one proved 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 levelled the match after breaking for a 5-3 lead.

Djokovic ruthlessly picked Del Potro off in the decider, breaking in the third game with an angled forehand and again for 5-2 before wrapping 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."