PARIS -- To-be ATP world No 1 Novak Djokovic and Swiss legend Roger Federer advanced to the quarterfinals at the 2018 ATP Paris Masters here on Thursday.

Damir Dzumhur from Bosnia and Herzogovina pulled out in the second set due to lower back pains, when Djokovic was leading 6-1, 2-1.

Djokovic is set to become world No 1 for the first time in two years after Rafael Nadal withdrew due to an abdominal concern on Wednesday. Following Thursday's third-round win, the Serbian has extended his unbeaten streak to 20 matches.

In the quarterfinals, Djokovic will face fifth-seeded Marin Cilic, who saw off Grigor Dimitrov 7-6 (5), 6-4. Despite a 15-2 head-to-head record, Djokovic lost to Cilic in the same stage at the 2016 Paris Masters to cede his top position. Cilic needs to beat Djokovic on Friday for a guaranteed spot in the ATP Finals later this month in London.

"It's going to be a tough one. Tough to receive his missiles. His first serves are really fast, and he uses a lot of rotation and variation with his serves. He can play very aggressively and also defend well," said Djokovic.

In his first match at the tournament in three years, Federer embraced warm welcome from home crowds in the Bercy Arena. The Swiss veteran didn't let them down as he accounted for Fabio Fognini in straight sets.

After two players held their serves in the first two games, Federer registered his first break throughout the match and built a 4-1 lead, seeing his Italian rival throw the racquet onto the court in discontent.

That proved to be of help for regrouping Fognini later on with two games in a row. He went on to trail 4-5 while leading 40-15, but let slip his advantage in the tenth game to lose 6-4 in the first set.

Federer continued his momentum for another break midway through the second set, before sealing his quarterfinal spot 6-3 following Fognini's double faults.

"I served better than him at important moments. This was actually the key of the match," said Federer.

"What is crucial in a match is who serves better. It was me today. He didn't start well as well, which is always a bad thing for him," he added.

Federer will take on Kei Nishikori, who marched past No 7 seed Kevin Anderson 6-4, 6-4. The Japanese kept his London hope alive with the win, but he needs to reach the final for a chance.

John Isner, however, is set to miss the Finals after losing to Karen Khachanov in a tussle featuring two tie-breaks in almost two and a half hours.

Despite saving three match points before triumphing the tie-break 11-9 in the second set, the American serving machine failed to repeat that in the decider as the Russian sealed the win at 10-8.

Khachanov booked a quarterfinal encounter with No 4 seed Alexander Zverev, who got past Diego Schwartzman 6-4, 6-2.

Defending champion Jack Sock shrugged off lucky loser Malek Jaziri from Tunisia 6-0, 6-4, establishing a duel against the 2018 Roland Garros runner-up Dominic Thiem.

Thiem, on pace to qualify for the Finals, rallied past the Shanghai Masters finalist Borna Coric 6-7 (3), 6-2, 7-5.