Britain's Andy Murray and Japan's Kei Nishikori are other notable absentees due to injury.
Spain got off to a flying start in front of a sell-out 12,500 crowd in La Caja Magica, Madrid,
The revamped competition, featuring 18 nations in one venue for the first time, has been notable for long, sometimes chaotic days but also for exhilarating action.
The 32-year-old Murray, who returned from hip surgery in June, made heavy weather of beating 79th-ranked Dutchman Tallon Griekspoor on Wednesday.
Spain can reach the quarter-finals on Wednesday by beating Croatia, who axed captain Zeljko Krajan two days before their opening tie and will be without injured talisman Marin Cilic.