Novak Djokovic is feeling more optimistic after continuing his fine run at Wimbledon by reaching the quarter-finals.

The Serbian, seeded 12th at the All England Club, has dropped just one set on his way to a last-eight clash with Kei Nishikori.

Djokovic cruised past Karen Khachanov 6-4 6-2 6-2 on Monday, marking the second straight major at which he has reached the quarter-finals.

The 12-time grand slam champion, who has endured a difficult 12 months due to an elbow injury, said he was nearing his best form once more.

"Approaching Roland Garros and Wimbledon this year for me is quite different from any other year because of circumstances that I was in in the last 12 months, the injury, the surgery, so forth," Djokovic said.

"But I felt like in the last month and a half, the level of tennis has been very close to where I would like it to be, where I'm used to having it, so to say, and playing on."

Djokovic said the past year had been frustrating, but his recent performances have left him optimistic.

"I feel like the big challenge for me, obviously first of all after surgery was to really be able to be comfortable with the change of racquet, with different compensations that I've made in the motion of my serve," he said.

"My game overall was just disturbed. I didn't feel comfortable on the court for long time. Indian Wells, Miami, most of the clay-court season.

"I just had to go back to basics and hit as many balls as I can on the practice courts, just get that feel. Also psychologically obviously, I was so fortunate to have so much success on the tour over the course of 10-plus years.

"I was a top-three player for so many years in a row, it was quite a strange feeling for me not to be able to deliver my game that I know that I possess, that I know I've been delivering for so many years. It was frustrating, to be honest.

"But I had to trust the process. I'm still part of that process. But things are looking much better in the last month and a half."