"The World Tour Finals last year was one of my best experiences ever on the tennis court. "I was always dreaming of playing the World Tour Finals and I'm excited to be back there," the Serbian said.
Elsewhere, the 2005 Paris champion Tomas Berdych overcame big-serving South African Kevin Anderson 1-6 6-3 6-4 to secure his place in the quarterfinals.
Berdych, who will also play in London next week, will now face France's Gilles Simon after his opponent, Japan's Kei Nishikori, withdrew with an injury.
World No. 8 Jo-Wilfried Tsonga guaranteed his place in London after defeating Spain's 11th-ranked Nicolas Almagro 7-6 (7-4) 7-6 (7-3).
"It's really important for me to have qualified," said the Frenchman, who will next face world No. 5 David Ferrer following the Spaniard's three-set win over Stanislas Wawrinka.
"With the big four at the moment, it's not easy to get rewards in tennis, so for me this is one of my rewards. It feels good to be a part of it.
"I have some great memories from last year where I played the final against Roger. It was huge for me, one of the big moments of my career."
Canada's world No. 13 Milos Raonic was dumped out at the third-round stage after going down 6-3 7-6 (7-1) to Djokovic's conqueror, American Sam Querrey -- who will play Llodra in the last eight.