Australia's Groves doubles up at Vuelta
Alpecin-Deceuninck sprinter Groves came home ahead of Italy's Filippo Ganna while defending champion Remco Evenepoel stretched his overall lead.
Evenpoel stunned the peleton by winning the intermediate sprint, 11km from the finish line, to pocket six bonus seconds.
The Belgian now has an 11-second lead over Movistar's Enric Mas and is 17 seconds ahead of Lenny Martinez of Groupama-FDJ.
Tour de France champion Jonas Vingegaard, of Jumbo-Visma, is fourth overall, 37 seconds off the lead.
Groves, 24, controlled the sprint, resisting pressure from Ganna who was still able to collect 60 points to consolidate his hold on the green jersey for points leader.
The end of the stage was marked by a fall just over three kilometres from the finish which split the leading bunch.
Meanwhile, the start of the stage saw Jayco-AlUla team leader Eddie Dunbar forced to retire after a fall in the neutralised zone while Italy's Filippo Zana was also forced out with stomach pain.
On Thursday, the Vuelta moves back to the mountains with the sixth stage featuring a category 1 climb in the Sierra de Javalambre.