Paris - Roger Federer and Novak Djokovic will meet for the 47th time Saturday with a place in the Paris Masters final at stake.
Federer, chasing a 100th career title, moved a step closer to that goal with a 6-4, 6-4 win over Japan's Kei Nishikori in Friday's quarter-finals in the French capital.
Djokovic, who will take over as world number one from Rafael Nadal on Monday, reached the semi-finals after recovering from a break down in the final set to defeat Marin Cilic 4-6, 6-2, 6-3 and rack up his 21st consecutive win.
Djokovic leads Federer 24-22 and has not lost to the Swiss since 2015.
However, the Serb star admits that his challenge on Saturday may be compromised by a heavy cold.
"I'm still struggling. It's kind of a flu. It's not easy to perform every single day, the energy levels are not as high," said the second seed.
Four-time Paris champion Djokovic, who went into Friday's quarter-final with a 15-2 record over Cilic, had three break points in the opening set but the Croatian held firm.
Cilic wrapped up the opener in the 10th game, ending Wimbledon and US Open champion Djokovic's 30-set win streak in the process.
However, the 14-time major winner roared back, breaking twice in the second set to level the contest.
Cilic, who defeated 31-year-old Djokovic in the final at Queen's Club in the summer, was a break ahead again for a 2-1 lead in the decider.
But Djokovic fought back, retrieving the break in the next game and breaking again in the eighth to lay the foundation for victory.
The only consolation for Cilic was booking a place in the ATP Finals in London later this month courtesy of Nishikori's loss.
"It was a big challenge. I was a break down in the third set. This match could have gone either way," said Djokovic.
"I won the match, but if he would've won it, it would've been absolutely deserved."
Djokovic now has a 30-1 record since the start of Wimbledon with his only loss in that time coming against Greece's Stefanos Tsitsipas in the third round of the Toronto Masters in August.
Should he win the title on Sunday, Djokovic will equal Nadal's record of 33 Masters triumphs.
Federer, the 2011 champion in Paris, last beat Djokovic at the ATP Tour Finals in 2015 and was comprehensively beaten by his long-time rival in the Cincinnati Masters final on the eve of the US Open this year.
He was untroubled by Nishikori on Friday, breaking the Japanese in the ninth game of the first set and opening game of the second.
Saturday's other semi-final will feature Austrian sixth seed Dominic Thiem against unseeded Russian Karen Khachanov.
Khachanov, the winner at last month's Kremlin Cup in Moscow, demolished out-of-sorts fourth seed Alexander Zverev of Germany 6-1, 6-2 while Thiem knocked out defending champion Jack Sock, the 16th-seeded American, 4-6, 6-4, 6-4.
World number 18 Khachanov saved two match points against John Isner in the last 16 but needed just over an hour to dispatch Zverev, who dropped his serve six times in eight games.
Khachanov, 22, shrugged off complaints from Zverev over noise coming from the Russian player's box during the match.
"When you are playing and you are losing sometimes, you just get a little bit frustrated and upset," said Khachanov, who has reached the last-four of a Masters for the second time after a semi-final run in Toronto this summer.
"Everything is fine between us and there is no problem."
Thiem booked his place in the ATP Finals for the third consecutive year by knocking out Sock who will drop out of the world's top 100 as a result of his loss on Friday.
Thiem has won the second-most matches on the ATP Tour this year -- now 53-17 -- one behind Zverev's mark of 54-17.