Nick Kyrgios vents over missing towels in Stuttgart first round loss to Wu Yibing

The 25th-ranked Australian, who finished runner-up 11 months ago at Wimbledon to Novak Djokovic, was playing for the first time since undergoing knee surgery in January.

His mood turned sour in the first set, with complaints about the colour of the towels, the bounce of the grass court and a shortage of sideline water from an eco-dispenser.

But it was the coloured sponsor towels on court which got the Australian steaming.

"It took me three games to get white towels," he said after his loss to the Chinese player who won the Dallas ATP title in February.

"They soak up the sweat better. I'm the same at every tourney. They all have white ones in the locker room.

"I'm not asking to move mountains. White towels actually take the sweat off my body - we are professional athletes."

Kyrgios said that he still has work to do on his game and fitness after playing his first tour-level match since last October in Tokyo.

"It was my first match in a long time," he said. "I guess it was my final task of rehab to go out, play matches and see how my knee pulled up.

"Credit to him, he played as good as he needed to beat me."

Kyrgios added: "I have to get used to playing in front of the crowd again. Feeling the moments like break points - all that stuff is new to me again.

"I made it through the match with no major pain. I've got a lot more work to do, but I'm taking some positive signs from it."

The loss still featured 15 aces from Kyrgios, with Wu hoping to improve his own serve as he gets more acclimatised to grass.

"I'm very happy to have played Nick in the first round, he's one of the best players on grass," the winner said.

"The key for me was to stay cool, stay patient and find a way.

"He has a better serve than me, but I was more patient. I will work to improve my serve."

Kyrgios is due to compete next week on grass in Halle, Germany, where he lost last year in the semi-finals.