Russia are currently five points clear at the top of Group F with a perfect record from their first four matches, but Capello is expecting a tough test at Windsor Park.
The former England boss knows how much of a threat Northern Ireland can be on their home ground and has told his players to sharpen their focus.
"I believe it will be the most important match for us," the Italian told the Rossiiskaya Gazeta daily.
"Northern Ireland have held Portugal to a 1-1 draw last year. For our team the away clash at Belfast will without doubts be the most serious test.
"Northern Ireland are very dangerous on the counter-attack and in dead ball situations. I expect a very tough clash on Friday."
Israel and Portugal are the nearest challengers to Russia and Capello insists there is a lot of hard work still to do to secure a World Cup spot.
"The five-point lead doesn't mean anything," he said. "We have played just four matches and the most serious clashes are still ahead. I expect tough opposition in every one of the remaining matches.
"We started well and left our rivals behind. But the qualifying tournament is a long way to go and we have no right to relax until we win our place in the World Cup finals."
Capello added that Russia would probably play their home qualifiers with Luxembourg and Israel in September in St Petersburg's Petrovsky stadium, the home of reigning Russian champions Zenit.
"Luzhniki Stadium, the 2018 World Cup main venue, will be closed for renovation, while the pitch at Moscow's Lokomotiv arena was in poor condition when we played our last qualifier there," Capello said.
"I think it's a good idea to play at St Petersburg in September. The arena there is very comfortable with an excellent pitch. It seems to be a good choice for us."