Grigor Dimitrov is out and Stan Wawrinka is in the Sofia field.

Defending champion Dimitrov was forced to pull out of his home tournament, which begins next week, due to a shoulder injury.

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"I and my team had to take one of the most difficult decisions—to skip this year's edition of the tournament in Sofia," Dimitrov said in a statement. "Earlier today I had my last medical consultation. Unfortunately I will not be at 100 percent ready for next week. If I play it would lead to additional risks and complications."

The tournament announced Wawrinka, Dimitrov's former doubles partner, has stepped into the field.

"I am really happy to come for the first time in Bulgaria, in Sofia," Wawrinka told the tournament website. "I’ve heard a lot of good things about the tournament. I came back from Australia with a lot of work to do for my knee, for my tennis-wise. I felt better this week, so I decided that it may be a good option for me to start a little bit earlier with tournaments."

The 32-year-old Swiss, who underwent knee surgery last year, made his return at last month's Australian Open. Wawrinka, who defeated Ricardas Berankis in the opening round before bowing to Tennys Sandgren in round two, said his knee has improved since Melbourne.

"For sure. I am feeling much better," Wawrinka said. "Since the first day I came back from Australia I was working really hard physically and also for my tennis. I am feeling better and better and starting to come back to my top level. I am happy with all the improvement I am making and I am happy to back in the tour."

Wawrinka cautioned it will take time for him to regain his top level.

"I always come into tournament to play my best tennis. I am not even there yet," Wawrinka said. "That is not the goal you are putting in your head before a tournament. It is to be ready for the first match. You know I didn’t play for six months and I had only one tournament in Australia. So I think it isn’t easy. I know what I can do, I know what my level can be when I am playing good tennis. But first of all I am really happy to be back on the Tour and to come to Sofia for the first time."

Photo credit: @StanWawrinka