The Vienna ATP event said on Thursday it had added world No 3 Carlos Alcaraz to the tournament's line-up, pouncing when a Saudi Arabian exhibition event was cancelled.
"When indications emerged on Wednesday that the Six Kings Slam... would be postponed until the end of December, tournament director Herwig Straka reacted immediately and contacted Carlos Alcaraz's management," Vienna organisers said in a statement.
"Everything happened very quickly after that, and Carlos agreed to come to Vienna without much hesitation," said Straka.
"His commitment is a tremendous honour for our tournament."
The lucrative Six Kings Slam exhibition, originally scheduled for October 21-24 and to be streamed live by Netflix, was postponed after Yemen's Houthis carried out deadly attacks on the kingdom.
Organisers now hope to hold the tournament on December 27-30.
US Open champion Alexander Zverev, Alex de Minaur and Learner Tien had already planned to play in Saudi Arabia and then in Vienna, which starts on October 26.
Further top-10 players already announced for Vienna, an indoor, hard-court ATP 500-level event, are Daniil Medvedev, Flavio Cobolli and Frances Tiafoe.
Alcaraz only previously played the Vienna tournament in 2021, when he reached the semifinals.
The former world No 1 has shown inconsistent form since returning from a long injury layoff at the US Open at the end of August.
He won in Tokyo in September but was knocked out in the third round of the Shanghai Masters in straight sets by Zizou Bergs on Monday.