Medvedev served as president after Putin stepped aside following the maximum two consecutive terms allowed by the constitution after his 2000-2008 stint.
But Putin, 60, stayed on as a powerful prime minister and Medvedev, 47, never fully emerged from the shadow of his fellow Saint Petersburg native, an impression strongly reinforced when Putin returned to the Kremlin in May 2012.
"This (returning to the presidency) depends on a whole range of factors," Medvedev said.
"Never say never, especially as I swam in that river once and this is a river that you can swim in twice," he said.
Some of Medvedev's supporters -- who saw him as a possible champion of a refreshed, innovative and more pro-Western Russia -- were hugely disappointed by his apparent surrender of the Kremlin to Putin.
But Medvedev played up the tight links between the two men, saying: "I would hardly have become prime minister under another president, I cannot imagine it at all."
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