Nadal will face Djokovic in the semi-finals on Saturday
Rafael Nadal's win over Jo-Wilfried Tsonga at the Paris Masters earned fellow Spaniard Fernando Verdasco a place at the World Tour Finals.
The world number two came through 7-5 7-5 against the defending champion.
Tsonga needed to win the title to stop Verdasco qualifying for the season-ending event in London for the first time in his career.
Earlier, Novak Djokovic ended Robin Soderling's chances of overtaking Verdasco with a 6-4 1-6 6-3 win.
Djokovic and Nadal will meet in the semi-finals on Saturday.
In Friday's other quarter-finals, Radek Stepanek went through when US Open champion Juan Martin del Potro, who had undergone treatment for a stomach problem earlier in the tournament, retired with an injury at 4-0 in the first set, and Gael Monfils beat Marin Cilic 3-6 6-4 6-4.
The line-up for the World Tour Finals, which begin on 22 November at the O2 Arena, is now complete and will feature: Roger Federer, Nadal, Djokovic, Murray, Del Potro, Andy Roddick, Nikolay Davydenko and Verdasco.
Tsonga won the Paris title in 2008 to snatch qualification for last year's season finale, and might have had high hopes of repeating the trick as he faced Nadal, who had come from match points down in both his previous matches.
But the Spaniard, who still has an outside chance of replacing Federer as world number one at the end of the season, was in much better form on Friday, saving all six of the break points he faced.
"It was a very important victory for me," said Nadal, who returned to competition in October after a month off with a pulled stomach muscle. "It's probably the first big victory after I came back from injury."
Nadal, whose last title was at the Rome Masters in April, admits Djokovic is the form player in men's tennis after his victory over Roger Federer in the Swiss Indoors final last week.
"He is the best player in this part of the season," said Nadal, who has a 14-5 win-loss record over the Serb. "It's going to be a good test. I know he is the favourite on this kind of surface."
Soderling, meanwhile, will be kicking himself failing to take his chances against a below-par Djokovic.
The Swede had nine break points in the first set, but could take only one - when he drew level at 4-4.
However, three unforced errors in the very next game allowed Djokovic to break straight back and the Serb served out the set.
Soderling, who had lost all four of his previous matches against Djokovic, dropped only 11 points as he rattled through the second set and he had chances for an early break in the third.
But the world number three dug deep to see off the danger and Soderling blinked first in game eight.
"I'm very pleased to go through, it was a big struggle," said Djokovic.
"Mostly I was fighting myself today. In the second set I was not moving well and I was letting him control the match but in the end I managed to hold the nerves and focus."