Best was included in the starting line-up named on Wednesday, but has been receiving intensive treatment on the shoulder injury sustained during Sunday's victory over Italy.
"Rory's come up great and is fine to start the game," said defence coach Les Kiss.
Best sprained the AC joint on his right shoulder at Otago Stadium and was locked in a race against time to prove his fitness for Ireland's attempt to reach the semi-finals for the first time.
"We obviously had some precautions in place ready to cover him if he failed to pull through, but he's in tip-top shape so full credit to him and the medics," added Kiss.
Best's outstanding form as part of a highly-impressive Ireland front row compelled coach Declan Kidney to do everything possible to ensure his inclusion.
Number eight Jamie Heaslip is delighted the Ulster front row has been passed fit.
"It's great Rory's available because he's one of the senior players in the team," said Heaslip.
"He's an influential player and his darts in open play are great."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.