By Lidia Kelly and Michelle Martin MOSCOW/BERLIN (Reuters) - Mikhail Khodorkovsky, the Russian oil tycoon jailed for a decade after posing a challenge to Vladimir Putin, was freed by a presidential pardon on Friday and immediately flew to Berlin where he hoped to be reunited with his family. Once Russia's richest man, the 50-year-old looked pale and thin but happy in a photograph of him being greeted by German well-wishers on the tarmac after landing on a private jet. President Putin, who surprised Russians and gave a brief lift to the stock market by announcing Khodorkovsky's pardon on Thursday, said he was acting out of "principles of humanity" because Khodorkovsky's mother was ill. A Russian government source said freeing his best-known and potentially most powerful critic could deflect international complaints about Putin's human rights record as Russia prepares to host the Winter Olympics at Sochi in seven weeks' time.