The striker was diagnosed during a pre-season training camp in July and had been signed only weeks earlier by Dortmund for an initial fee of 31 million euros ($33.24 million).

Haller scored 34 goals for Dutch champions Ajax Amsterdam last season including 21 in the league and 11 in the Champions League.

However, he has been out of action since July following the discovery of the tumour. The Ivory Coast forward has had surgery and chemotherapy for the past few months, returning to training last week.

"Of course, it's been a tough six months," Haller told reporters from Dortmund's training base in Marbella, Spain on Monday. "Obviously I am happy to be here with the guys and able to play in this great weather and these facilities."

Haller, who previously played for Premier League club West Ham United, is hoping to be fully fit in time for the season restart in less than two weeks when Dortmund host Augsburg in their first league match of the year on January 22.

"Of course everything is possible," Haller said. "I have no restrictions in my mind. The only people who can give me restrictions are the coach and the medical staff. I will do my best to play on the 22nd."

Dortmund are sixth in the Bundesliga, nine points behind leaders Bayern Munich.