Security officials promptly made a human shield around the players as some others dragged the person, who appeared to carry a banner reading “abolish refugee detention”, off the court at the Rod Laver Arena.
Nadal was attempting to save a breakpoint while serving at 5-3 up in the second set as the person fell awkwardly from the stands behind Medvedev’s end of the court, halting play for a few minutes.
The Spaniard, who is chasing a 21st Grand Slam title, went on to lose the game and the second set in a tiebreaker.
Earlier this month, protesters criticised Australia’s harsh asylum policies when top-ranked Novak Djokovic was held in an immigration detention hotel after his visa was revoked on his arrival at Melbourne.
The Serbian was eventually deported from the country after a court upheld the government’s decision to cancel his visa, capping days of drama over Australia’s Covid-19 entry rules and his unvaccinated status.