Engineers drilling to reach 33 miners who have been trapped underground in Chile for over two months say they have just over 100 metres (328ft) to go.
But the main drill has had to slow down because it is passing close to a mine tunnel, with a risk of collapse.
The engineers expect to reach the trapped miners in the next few days.
Expert mine rescuers and navy medics, who will go down the shaft to help the miners out, have begun rehearsing the operation.
The drill that is closest to the miners - the T-130, known as plan 'B' - has now penetrated 520m (1706ft) of the 630m of rock separating them from the surface.
Operations were briefly stopped so the mine hammer could be replaced.
It must now pass close to a mine shaft and through a geological fault line, which means drilling will have to proceed more slowly.
The head of the rescue operation, Andre Sougarret, played down speculation that the drill might reach the miners at the weekend.
"There are still 100 metres to go. We are approaching a difficult rock layer, as we know from earlier drilling, so it is premature to speak of Saturday," he said.
Once the tunnel is complete, engineers will have to decide whether to line the shaft with metal casing to make it more secure before lowering the specially-designed rescue capsule that will pull the men to the surface.
If they decide not to line it, it will speed up the operation.
Mr Sougarret said safety would remain the prime consideration.
"We have to wait until we break through. Then we will check the shaft, what state it is in, what risks we face, what the risks of installation are, and on that basis we will take the decision on putting in the casing," he said.
A 16-strong team of firefighters, mining rescue experts and Chilean navy medics is standing by to conduct the operation.
Teams of two will be working in 12-hour shifts to go down the shaft and help the miners use the rescue capsule.
The leader of the team, Rene Aguilar, said they had all the necessary expertise.
"The majority are specialists in underground rescue, in rescuing trapped people. Many are also firefighters as well as miners, members of the Chilean fire corps. We also have mountaineers, and there is great underground experience," he said.
It is expected to take at least an hour to pull each of the trapped men to the surface.
Officials say everything needed for the rescue is now in place.
Huge cranes have been brought in to lower the metal casing if it is needed.
In a final detail, sunglasses have been brought in to protect the men's eyes when they emerge into the sunlight after two months underground.
Miners' relatives, who have been camping at the site for the past two months, are preparing a party to celebrate their rescue.
Hundreds of journalists from around the world are expected to cover the event.
The men were trapped by a rockfall at the mine near Copiapo, about 725km (450 miles) north of the Chilean capital Santiago, on 5 August.
Rescuers had almost given up the search when they located the miners 17 days later.
They have been trapped underground longer than any previous group.