Australia's Bernard Tomic was forced to retire in the first set of his opening-round match at the ATP World Tour event in Dubai.
Tomic, a quarter-finalist in Marseille last week, was trailing Romanian Victor Hanescu 3-2 when he chose not to continue.
World No.45 Tomic was suffering from what is being reported as a "general illness".
Meanwhile, Novak Djokovic trounced fellow Serb Viktor Troicki 6-1 6-4 in his first-round match as the world No.1 extended his unbeaten run to 14 matches.
Djokovic, whose winning streak stretches back to October and includes claiming a third straight Australian Open title, was broken once in the second set but was otherwise untroubled in his first match since Davis Cup duty in early February.
"That was a great match, especially in the first set, considering the fact I haven't played an official tournament for three, four weeks," said Djokovic.
"I wanted to be extra focused on the court today, because I also knew he has a big serve, as he had proven tonight, and can be very dangerous on these courts. So you need to be really committed to every point and play well."
Former US Open champion Juan Martin Del Potro saved three match points on his way to beating Marcos Baghdatis 4-6, 6-4, 7-6 to make the second round.
Fourth seed Del Potro recovered from 4-1 down in the deciding set, with the Argentine's superior ground strokes ultimately wearing down his Cypriot opponent as dusk fell.
"Marcos also deserved to win but the luck was behind me and I played better in the big moments," Del Potro said in a courtside interview. "In the tiebreak I was solid."
Fifth-seeded Frenchman Jo-Wilfried Tsonga was beaten by his Davis Cup teammate Michael Llodra 7-6 (7-3) 6-2.
Tsonga was back on court less than 48 hours after winning in Marseille with depleted energy and, as the match wore on, declining motivation too.
Earlier, Russia's Nikolay Davydenko routed world No.9 Janko Tipsarevic 6-0 7-5 in sweltering conditions.
The first two games took 31 minutes to complete, with Tipsarevic going through three rackets in the opening three games.
The Serb came out fighting in the second set and broke twice for a 4-1 lead only for 31-year-old Davydenko to win six of the last seven games.
Third seed Tomas Berdych of the Czech Republic strolled past wild card Rajeev Ram 6-1 6-3.