Conversation so far:
Was the Giro-Vuelta double a race with Eddy Merckx?
Eddy Merckx decided to race the Vuelta a España and the Giro d'Italia, instead of racing the Tour de France.
Did Eddy Merckx do well in the Vuelta race?
Eddy Merckx won the opening prologue of the Vuelta to take an early lead, winning a total of six stages on his way to his only Vuelta a España title.

Question: Who did he race against?
Self-contained rewrite: Who did Eddy Merckx race against on his way to his only Vuelta a España title?