Novak Djokovic celebrated his return to the summit of world tennis with a stroll into the quarter-finals at the Paris Masters after Damir Dzumhur had to retire with injury in the second set of their third round clash on Thursday.
The Serb, who learned on Wednesday that he would be back as world number one next week following Rafa Nadal's withdrawal from the tournament through injury, was leading 6-1 2-1 when the Bosnian succumbed to a back problem that had flared up in the first set.
Djokovic extended his winning streak to 20 straight matches as he maintained the remarkable spell of form that helped him rise from number 22 in June to the top of the game again.
"Reflecting on what I've been through in the last year, it's quite a phenomenal achievement. And, of course, I'm very, very happy and proud about it," Djokovic told reporters.
Dzumhur suffered a tweak to his lower back when 4-1 down in the opening set.
Clearly in pain, he battled on in an 11-minute sixth game which eventually led to him being broken for a second time and, though he continued after receiving physio treatment in a time-out and taking pain-killers, there was no way back on Bercy's centre court.
Djokovic, looking in much sharper form than his opening match against Joao Sousa, took the set in 39 minutes and, once he had earned an inevitable break for 2-1, consoled the Bosnian, who told him at the net that he was in no shape to continue.
Seeking a record-equalling 33rd Masters and record-extending fifth Paris title, Djokovic will face a much more challenging test in the last eight against Marin Cilic, who had earlier looked in strikingly good form when beating Grigor Dimitrov 7-6(5) 6-4.
"He (Cilic) likes playing indoors. Obviously he has a big serve and big game from the back of the court, it's just a very powerful style," said Djokovic.
"It's going to be a tough one. Tough to receive his missiles. But I know his game. I've played him many, many times and we practise a lot. We respect each other. We get along very well. It's going to be a great match."
Cilic, who moved closer to securing one of the remaining places for the ATP Finals in London, beat Djokovic here at the equivalent stage two years ago and also at this year's Queen's final before Wimbledon.
"I am very close to London, to securing my spot mathematically," Cilic said. "Against Novak, it will be another big challenge. It's great to play him again."
John Isner's hopes of also making the London finals received a potentially fatal blow when he was beaten 6-4 6-7(9) 7-6(8) in by rising Russian Karen Khachanov.
The giant American saved four match points before the 22-year-old Khachanov, who had already staved off two himself, finally threaded a backhand winner past the net-rushing Isner to win after nearly two-and-a-half hours of massive hitting.
In the quarter-finals, Khachanov will meet Germany's fourth seed Alexander Zverev, who subdued the scurrying defiance of Argentina's Diego Schwartzman 6-4 6-2.
In a high-quality contest between two others seeking a London berth, Dominic Thiem, back in the city where he reached the French Open final, ended Borna Coric's slim hopes while boosting his own with a 6-7(3) 6-2 7-5 win.
The Austrian will next face a rejuvenated Jack Sock after the defending champion, who has endured a dramatic slump in the year since, outplayed Tunisian lucky loser Malek Jaziri 6-0 6-4 to win back-to-back matches for the first time in 2018.