"Flattening the curve was our first objective. We're getting close. But I ask everyone for sacrifice and resistance…
"The next objective is to reduce infections even more until the number of new contagions is lower than the number of people recovering each day," Anadolu Agency quoted Sanchez as saying in a televised address on Saturday.
Spain has the second most COVID-19 deaths and cases in the world. On Saturday, Spanish government data showed that 11,744 people have died from the disease, while 57,612 have been hospitalised, and more than 124,700 cases have been confirmed.
The original practice of quarantine began during the 14th century when ships arriving in Venice from ports suffering from the plague would have to wait for 40 days at dock.
Spain's quarantine, which began on March 14, will now last at least 45 days.
Spain has one of the strictest quarantines in the world. Since the beginning, no one has been allowed to leave their homes for anything non-essential.
Unlike many countries, outdoor exercise is not permitted.
"I understand how difficult it will be to stay home in isolation for two more weeks," said Sanchez.
The government, which can only call a state of emergency for 15 days at a time, previously extended the lockdown until April 12.