Davydenko is the seventh player to book his place at the London event
Russia's Nikolay Davydenko booked his place at the World Tour Finals in London despite losing to Robin Soderling at the Paris Masters.
Soderling remains hopeful of qualifying after the 6-3 3-6 6-4 win, earning a quarter-final against Novak Djokovic.
But Fernando Verdasco's defeat by Marin Cilic put Davydenko through.
Rafael Nadal is moving in on the world's top spot again after surviving match point against Tommy Robredo to edge into the quarter-finals in France.
Following Roger Federer's defeat on Wednesday, second seed Nadal overcame Robredo serving for victory at 5-4 in the deciding set before finally coming through 6-3 3-6 7-5.
Nadal, who saved five match points in the previous round against Nicolas Almagro, said: "I was a little less lucky than yesterday.
I will only be qualified if I win this tournament, so I'm not thinking about London
"He served for the match and it was in his hands, but I won anyway. I wasn't playing my best, but I played better than yesterday."
The Spaniard will now face defending champion Jo-Wilfried Tsonga of France for a semi-final berth.
Tsonga, along with Verdasco and Soderling, is fighting for the final place at the season-ending London event.
Verdasco will take it if Soderling does not make the Paris Masters final this week, and Tsonga fails to defend his title.
"I'm not here for the Masters, I'm here for Bercy," Tsonga said after brushing aside fellow Frenchman Gilles Simon 6-2 6-3 to reach the quarters.
"I will only be qualified if I win this tournament, so I'm not thinking about London."
Fernando Gonzalez's hopes of reaching London ended in dramatic fashion when he squandered seven match points against Juan Martin del Potro before retiring with a knee injury.
In the day's final match, the Chilean took the first set on a tie-break but after seeing chances to take the match slip by in the second, he retired after losing the tie-break.
Del Potro will now play Radek Stepanek after the Czech's surprise three-set win over Andy Murray.
Third seed Djokovic cruised into the final eight with a 6-2 6-2 victory over French qualifier Arnaud Clement.
Julien Benneteau failed to hit the heights he reached in Wednesday's stunning win over top seed Federer as he lost 6-4 6-3 to Gael Monfils.
Monfils meets Cilic in the last eight after the Croat's 3-6 6-3 6-4 win over Verdasco.