Paris - Rafael Nadal pulled out of the Paris Masters on Wednesday guaranteeing Novak Djokovic's return to world No 1 spot.
Announcing his withdrawal at a hastily arranged press conference Nadal said: "My abdominal muscles haven't felt good for the past few days, especially on serve, doctors recommended that I didn't play."
Djokovic arrived in Paris knowing that he would regain the top spot he relinquished two years ago if he bettered the Spaniard's performance at the indoor event he has won four times.
The Wimbledon and US Open champion becomes the first player to start the season outside the top 20 and end up top of the ATP rankings since Marat Safin in 2000.
The 14-time Grand Slam champion had begun his pursuit of the top spot in encouraging fashion with a 7-5, 6-1 defeat of Joao Sousa on Tuesday, his 19th consecutive win.
For Nadal it means delaying his long awaited comeback from injury which has kept him sidelined since the US Open.
Nadal was forced to withdraw from his US Open semi-final against Juan Martin del Potro in early September with a knee injury.
The problem sidelined him for Spain's Davis Cup semi-final against France, and also from tournaments in Beijing and Shanghai.
"It's been a difficult year for me in terms of injuries, and so I'd prefer to avoid doing anything drastic," he told the media minutes before his intended match against compatriot Fernando Verdasco.
"It wouldn't have been good for me nor anyone to return to the court knowing that I wouldn't be able to play my best.
During his enforced time out Nadal joined in the clean-up operation after flash floods killed 12 people on the Spanish holiday island of Majorca where he lives.
After Paris the ATP tour's eight top-ranked players congregate in London from November 11 for the season's Finals.
The 32-year-old Nadal has enjoyed a successful year, clinching his 17th Grand Slam title by winning an 11th French Open in June, after triumphs in Monte Carlo, Barcelona and Rome.
But it is Djokovic who will be top dog when the latest ATP rankings appear on Monday, irrespective of his fate in the French capital this week.
Despite the blow the tournament organisers will be relieved to see Roger Federer making his first appearance at the competition since 2015.
The Swiss great starts his quests for his 100th ATP title in the last 16 on Thursday after his Wednesday rival Milos Raonic withdrew through injury.
World number three Federer will face Italy's Fabio Fognini, ranked 14, who made the next round when Hungary's Marton Fucsovics withdrew injured.
Elsewhere, German fourth seed Alexander Zverev, playing his 70th match of the season, defeated Frances Tiafoe of the United States 6-4, 6-4 while defending champion Jack Sock put out Richard Gasquet 6-3, 6-3.
South African seventh seed Kevin Anderson, the champion in Vienna at the weekend, battled past Nikoloz Basilashvili of Georgia 6-3, 6-7 (3/7), 7-6 (7/3).
Anderson next faces Japan's 10th seed Kei Nishikori, who he defeated in the Austria final. Nishikori made the last-16 with a 7-5, 6-4 win over Adrian Mannarino of France.
Bulgarian ninth seed Grigor Dimitrov, who is working with US legend Andre Agassi in Paris, went through with a 7-6 (12/10), 6-4 victory over Spain's Roberto Bautista Agut.
World number eight Dominic Thiem boosted his chances of qualifying for the ATP Finals, beating Gilles Simon 6-4, 6-2 and next faces Borna Coric for a quarter-final slot.
Results from the ATP Paris Masters on Wednesday (x denotes seeding):
Malek Jaziri (TUN) bt Fernando Verdasco (ESP) 7-6 (7/5), 1-6, 6-3
Jack Sock (USA x16) bt Richard Gasquet (FRA) 6-3, 6-3
Borna Coric (CRO x11) bt Daniil Medvedev (RUS) 6-4, 6-4
Dominic Thiem (AUT x6) bt Gilles Simon (FRA) 6-4, 6-2
Alexander Zverev (GER x4) bt Frances Tiafoe (USA) 6-4, 6-4
Diego Schwartzman (ARG x15) bt Feliciano Lopez (ESP) 6-4, 3-6, 7-5
John Isner (USA x8) bt Mikhail Kukushkin (KAZ) 6-3, 6-7 (2/7), 7-6 (7/1)
Kevin Anderson (RSA x7) bt Nikoloz Basilashvili (GEO) 6-3, 6-7 (3/7), 7-6 (7/3)
Kei Nishikori (JPN x10) bt Adrian Mannarino (FRA) 7-5, 6-4
Fabio Fognini (ITA x13) bt Marton Fucsovics (HUN) - walkover
Roger Federer (SUI x3) bt Milos Raonic (CAN) - walkover
Grigor Dimitrov (BUL x9) bt Roberto Bautista Agut (ESP) 7-6 (12/10), 6-4