Novak Djokovic struggled to a three-set victory over Xavier Malisse at the Swiss Indoors in Basel.
Djokovic, 24, beat the Belgian 6-2 4-6 7-5 in two hours and 12 minutes.
It was the world number one's first ATP match since winning the US Open in September, soon after which he suffered a back injury in a Davis Cup tie.
The victory takes his season record to 65-3 as he seeks a winning end to a year that has seen him claim three Grand Slam titles.
Djokovic had warned that he did not expect to play at his full potential but broke Malisse at the first opportunity.
Malisse fought back in the second and required a medical break after colliding with the umpire's chair but in the third set the Serbian quickly closed out the final two games after some initial uncharacteristic errors.
Mardy Fish's hopes of qualifying for the ATP World Tour Finals were dealt a blow after he retired from his first-round match with fellow American James Blake.
A hamstring injury meant 29-year-old Fish, who is currently in the eighth and final qualifying spot for the end-of-season event in London, could manage just one game of the match.
Fish, who has never qualified for the ATP Tour Finals, overtook Andy Roddick as the number one American after winning 20 of the 26 matches he has played since July.
He retains a 500-point lead over Spaniard Nicolas Almagro who is ninth in the world rankings.
Serbian Djokovic, Rafael Nadal and David Ferrer of Spain, Britain's Andy Murray, and Swiss Roger Federer have already qualified with three places still open.
Following his walkover, Blake will next play Russian Mikhail Kukushkin, who entered the field after Austrian Jurgen Melzer pulled out with a back injury.
National favourite Stanislas Wawrinka beat Ivan Dodig of Croatia 6-4 6-4, while countryman Michael Lammer beat Mikhail Youzhny of Russia 6-4 6-3.
The Swiss duo's victory comes a day after Roger Federer won his first-round match in his hometown.
Murray opens his account on Wednesday when he plays Robin Haase of Netherlands not before 1500 GMT.
The world number three is also scheduled to play doubles with brother Jamie against Max Mirnyi and Daniel Nestor later in the day.