* Germany in a hole after injury
* Monaco gives Argentina 2-0 lead (Adds Monaco win, Mayer quotes)
By Federico Coronado
BUENOS AIRES, Feb 1 (Reuters) - German number one Philipp Kohlschreiber pulled out when 5-4 ahead in the fifth set of a gruelling first rubber to hand Argentina the lead in their Davis Cup first round tie on Friday.
The world number 19 led 6-3 5-7 6-2 4-6 5-4 against Carlos Berlocq, a stand-in for the absent Juan Martin del Potro, when he pulled his left hamstring after a four-hour marathon played in more than 35 degrees heat.
World number 12 Juan Monaco then beat Florian Mayer 6-7 6-3 6-3 6-4 to put Argentina 2-0 up on the first day on the claycourt at Parque Roca.
"I can't move my leg. I can't take a single step," Kohlschreiber told German reporters in the privacy of the locker room as he waited for a pair of crutches to be able to leave the stadium on the outskirts of Buenos Aires.
"It was a bitter thing for me and the whole team," added the 29-year-old. "It's hard to assess the gravity of the injury until further studies are conducted."
Team captain Carsten Arriens will now have to make a tough decision about who to play alongside Christopher Kas in Saturday's doubles.
Kas was to have teamed up with Kohlschreiber against David Nalbandian and Horacio Zeballos. Arriens now has to choose between Mayer and Davis Cup newcomer Tobias Kamke.
"It's no secret Philipp's already leaving for Germany on crutches ... it's a very difficult situation for us," Mayer told a news conference.
Argentina, normally favourites at home, had started the tie as underdogs given Del Potro's decision not to play the Davis Cup this year to concentrate on the tour and a six-month injury layoff that has left Nabandian short of match practice.
"I can't believe I'm experiencing this," an elated Berlocq, ranked 70, said in a courtside interview.
"The whole team had drummed into me that I could win and at times I looked at the clock and thought 'I'm doing well because I'm tiring him and he has to play all three days'.
"Thank God, I got another chance after the defeat against (Tomas) Berdych," added Berlocq, whose loss to Berdych gave Czech Republic their semi-final victory at Parque Roca last September. (Writing by Rex Gowar in London; editing by Martyn Herman and Nick Mulvenney)