The 41-year-old Klose, who holds the record of 16 goals scored at World Cup finals, has joined Bayern's staff and will be next to head coach Hansi Flick on the bench on Sunday.

Lewandowski, 31, is enjoying the best season of his career, netting 25 goals in 23 league games and on course to better his 30-goal haul in both 2015/16 and 2016/17.

The league's top-scorer has used the time since the Bundesliga halted in mid-March, to prevent the spread of the coronavirus, to improve his fitness.

"I feel better than ever before because I was able to work hard on my physical fitness for the last two months," said Lewandowski, who underwent groin surgery in December.

The Bundesliga will become the first top European league to return this weekend during the pandemic with games behind closed doors.

Bayern head to Berlin with a four-point lead at the top of the table.

Mid-table Union have beaten previous league leaders Dortmund and Moenchengladbach this season, but the terraces at their compact east Berlin stadium will this time be empty.

Poland captain Lewandowski sent records tumbling by scoring in each of the first 11 league games this season.

He is now learning from ex-Germany striker Klose, who scored 53 goals in 150 games for Bayern during his career and last week signed a year-long contract to join the coaching staff.

"He was a great player and I know that he can help us with his experience and skills," added Lewandowski.

"You can learn something new from any coach to become a better player."

Union's Swiss coach Urs Fischer will not be in charge of the hosts for Sunday's game having broken the team's quarantine following a family bereavement.

Nevertheless, Bayern head coach Hansi Flick expects a tough match in Berlin.

"They're a team with a great mentality. We will face an aggressive team that takes the direct route to goal," Flick said of Union.

"We need to stand up and be counted. We can't get a result just through the quality of our football.

"We have to show our mentality and take on challenges."

Flick has plenty of selection options with only Philippe Coutinho, Corentin Tolisso and Niklas Suele still sidelined by long-term injuries.

The Bayern boss admitted he is unsure whether the squad has the match fitness for the 90 minutes.

"We had an internal friendly with three 20-minutes games on Sunday. The intensity was very high," he said.

"But we don't know whether the whole squad will be able to hold out over 90 minutes."

Due to the German league's hygiene guidelines, players on the Bayern bench will wear protective masks, while those on the pitch have been told to avoid handshakes or celebration hugs.

Flick said his stars are briefed on what to expect.

"Not much will change except for the wearing of masks in the dressing room," he said.

"We discussed with the players what they can expect.

"More than 200 countries will be watching, we have an enormously large audience so we want to deliver a top performance."