MELBOURNE: Japan's Kei Nishikori reckons he played some of his best tennis since returning from injury last year but it still was not enough to stop him from suffering a first-round exit at the Australian Open on Monday.

The former world number four, a four-time quarter-finalist at Melbourne Park, bowed out 7-5 7-6(4) 6-2 to Spanish 15th seed Pablo Carreno-Busta.

"Well, tough because I thought I was playing well but Carreno was playing better today," Nishikori told reporters.

"I thought I was playing pretty high level. For me, I played, I think, one of the best tennis so far, including last year after coming back."

Nishikori was the first man from Asia to reach a Grand Slam singles final when he finished runner-up at the 2014 US Open but he has struggled for consistency during an injury-plagued career.

He spent 10 months on the sidelines after undergoing elbow surgery and was slated to return at the 2020 US Open but pulled out after testing positive for the novel coronavirus.

Nishikori played some matches after returning in Austria in September but his season was again cut short by a shoulder injury he suffered during a five-set loss to Italian Stefano Travaglia at the French Open.

"I was kind of happy the way I'm playing with this level, but still not enough to beat this guy," Nishikori, who has slipped down to 42 in the ATP rankings, rued.