Rafael Nadal has pulled out of next week's season-ending Nitto ATP Finals in London because he is still struggling with an abdominal injury.
The Spaniard missed last week's Paris Masters with the same problem and has taken advantage of his enforced layoff to have ankle surgery.
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The 31-year-old, who was knocked off the No. 1 spot by Serbia's Novak Djokovic Monday, will be replaced by American John Isner at the Nov. 11-18 event at London's O2 Arena.
Writing on his Facebook page in Spanish, the Mallorcan said 2018 had been "a complicated year," which had been "very good" when he has been able to play "and in turn very bad" when it came to injuries.
His long-time press agent Benito Perez-Barbadillo emailed CNN a statement Monday saying Nadal was set to undergo surgery on a "free body bone" in his right ankle.
"An arthroscopy will be practiced to remove it and be able to be in perfect conditions for the beginning of next season," Nadal's doctor Angel Ruiz-Cotorro, of Clínica Mapfre de Medicina del Tenis, said in the statement.
His withdrawal is another blow for the organizer of the ATP Finals with two-time Wimbledon champion Andy Murray also missing with a long-term hip injury.
Nadal has played just nine events in 2018, winning five of them, including an historic 11th French Open title at Roland Garros for his 17th career grand slam.
He played his last match at the US Open in September, when he retired with a knee injury in the semifinal against Argentina's Juan Martin Del Potro.
It was his second retirement in a major match after an upper leg injury forced him out of the Australian Open quarterfinal against Croatia's Marin Cilic at the start of the season.