Third-seeded Roberto Bautista Agut joined sixth seed Philipp Kohlschreiber in the second round
Japan's Yoshihito Nishioka said his comeback from knee surgery will "take time", after letting a one-set lead slip in a first-round loss to Benoit Paire at the Dubai Championships on Monday.
The 22-year-old, playing in only his second ATP tournament since suffering a cruciate knee ligament injury in Miami last March, fell to a 5-7, 6-4, 6-1 defeat.
The world number 179, with a protected ranking of 66, admitted that while he fought hard, his level is not high enough to fully compete on a regular basis.
"I had to learn again how to move on court," he said. "For three months after the surgery I could not run -- I had to re-learn that again.
"I had to start from zero, do everything all over again."
Nishioka said his immediate goal is to return to his career-best ranking of 58th, after his first ATP Tour match since retiring injured in the first set of his second-round tie against Jack Sock at the Australian Open.
"It will take time, I cannot play three straight weeks," he added.
"I need to rest and then play, but I need to stay positive."
Nishikoka won a tight first set but eventually bowed to Frenchman Paire after losing the first five games of the decider.
"The second set was tough, he started playing more aggressive and moving better than he did in the first," said the Japanese number five.
"I could only keep trying and have faith in my game. In the third he didn't miss as much and I was getting tired, my movement was not good."
Third-seeded Roberto Bautista Agut joined sixth seed Philipp Kohlschreiber in the second round with a 6-3, 6-4 defeat of Florian Mayer.
The Spaniard stands behind top seed Grigor Dimitrov and French second seed Lucas Pouille at the Aviation Club for an event which lost Roger Federer at the last minute.
Bautista Agut's win ended a three-match losing streak which came after he won the Auckland title in January.
"I felt great on the court," he said. "He was mixing up all his game, and playing very aggressive on the return.
"I think I did a great job. Here the ball flies after a deep return, it wasn't easy to hold.
"It's a tough switch from indoors to outdoors. It was good for me to have three days of practice before this match."
Germany's Kohlschreiber, unable to defeat Andy Murray a year ago despite having seven match points, advanced over French qualifier Gleb Sakharov 6-4, 6-2.
"Everyone seems to be reminding me of that loss," Kohlschreiber said. "Losses are tough, they never feel good.
"It was a very entertaining match, with good atmosphere -- one of the best I've ever experienced.
"I'm happy with my performance today, I raised my game at the important moments."
The 33rd-ranked Kohlschreiber will next play Greek teenager Stefanos Tsitsipas, a winner over Kazakh Mikhail Kukushkin, 6-2, 6-7 (5/7), 6-3.
Dutchman Robin Haase, whose quarter-final loss to Federer in Rotterdam returned the Swiss to the top ranking at age 36, beat French qualifier Quentin Halys, while former finalist Marcos Baghdatis saw off Viktor Troicki.