Serbia's Novak Djokovic looks at the Halloween mask he was wearing when arriving at the court before his match against Belgium's Xavier Malisse during the Swiss Indoors ATP tennis tournament in Basel on Tuesday. Djokovic won 6-2, 4-6, 7-5. [Photo/Agencies]

BASEL, Switzerland - World No 1 Novak Djokovic won a hard fought three setter against veteran Xavier Malisse at the Swiss Indoors on Tuesday as he revealed he has recovered from what he termed the worst injury of his career.

Djokovic, playing for the first time in six weeks after a back injury in the Davis Cup semi-finals left him lying in agony on the court in Serbia, finally overcame the in-form Belgian 6-2, 4-6, 7-5 to progress to the second round.

The top seed, who took the Basel title in 2009 but lost the final a year ago to Roger Federer, walked onto the court wearing a Batman Joker Halloween mask, but got a fright of his own as he faced down Malisse, ranked 47 in the world.

The Serb ended with a break of the 30-year-old Belgian in the final game after losing his way as he served for victory leading 5-3 in the third set. Djokovic dropped serve on a fifth break point, his seventh double fault, but found a way to win three games later.

"I'm only just back from the biggest injury of my career," said the winner of 10 titles in 2011. "It was a big ask for me to have played really well

"I could do nothing for 4 1/2 weeks, I've only been hitting for a week-and-a-half. That's not enough to play at my best. I've lost rhythm.

"I had too many errors and too many double faults. But I'm happy just to be playing tennis again. I hope to build on this performance."

The loser of just two ATP matches this season - plus one in the Davis Cup semifinal against Argentina - improved to 65-3 on the season.

Serbia's Novak Djokovic wears a Halloween mask as he arrives before his match against Belgium's Xavier Malisse at the Swiss Indoors ATP tennis tournament in Basel, Nov 1, 2011. [Photo/Agencies]

Djokovic is the only man other than Federer to win the Basel title in the past five years.

The chances for Mardy Fish to play in his first ATP season finale may have been badly compromised as the American suffered a hamstring injury just 11 minutes into his opening match.

Fish was leading compatriot James Blake 1-0 in the first set when he was forced to retire.

"I didn't see anything. He suddenly started grabbing at his hamstring and had to call the trainer," said Blake.

"It's worrisome that it appeared to happen on one shot and not over the course of a few days.

"But Mardy is doing the cautious thing, he's gone for an MRI and we will have to see."

The retirement could mean the end of Fish's hopes of winning a place in the World Tour Finals in London from Nov 20.

The 29-year-old stands a provisional eighth in the race for the eight-man field, with three places still up for grabs.

Two Swiss players moved into the second round, with 2006 semifinalist Stanislas Wawrinka leading the way with his defeat of Croatian Ivan Dodig 6-4, 6-4. Qualifier Michael Lammer put out experienced Russian Mikhail Youzhny 6-4, 6-3.

Wawrinka will next face either second seed Andy Murray or Dutchman Robin Haase.

Serbia's Novak Djokovic sits beside his Halloween mask during a break of his match against Belgium's Xavier Malisse at the Swiss Indoors ATP tennis tournament in Basel, Nov 1, 2011. [Photo/Agencies]