While Aru has prepared for the Vuelta all summer after placing second at the Giro d'Italia, Nibali only added the race to his programme after he fell short of a podium finish at the Tour de France last month.
The Vuelta will be the first time the pair has ridden together in Astana colours since the 2013 Tour of Lombardy (they also both raced for Italy at the Ponferrada Worlds last year), but speaking at a joint press conference at Sestriere on Wednesday, they said that they would not race against one another.
"Fabio has prepared this race well and I think he could be the leader. I'll have the support of the team too, and if it turns out that way, I could work for him, certainly," Nibali said, according to Gazzetta dello Sport. "These are things that are decided on the road. I've raced with [Ivan] Basso in the past. Often you're stronger as a pair than you are alone."
Nibali's words were echoed by Aru, who found himself with an unexpected co-leader during the final week of the Giro, when Mikel Landa emerged to join him on the final podium in Milan. The Basque is also part of the remarkably strong Vuelta roster, though he is not part of the small group of Astana riders preparing for the race at Sestriere this week.
"There's respect between us. We're a group, a real team, and there's Landa too," Aru said. "A lot was said at the Giro, but there were no problems between me and him, and we achieved the best we could. The status of leader doesn't count for much."
Even beyond Astana's team of galacticos, there is an all-star line-up in store for this Vuelta, with the three podium finishers from the Tour – Chris Froome (Sky), Nairo Quintana and Alejandro Valverde (Movistar) – all set to start the race.
"I don't know how Froome will be, we'll have to see how he recovers," Nibali said. "Apart from at the end, Quintana and Valverde didn't risk much at the Tour, they rode on the defensive, to defend their third place. I don't know how they'll ride. [Domenico] Pozzovivo could be the surprise."
The Vuelta will also mark Nibali's first time encountering Froome since their clashes at the Tour last month. At Le Havre, Nibali incorrectly blamed Froome for bringing him down in a crash, while at La Toussuire in the final week, Froome complained that Nibali had launched his winning attack while the maillot jaune was rectifying a mechanical problem.
"We haven't seen each other since the Tour. There are lots of incidents in races, and what happens in the race stays in the race," Nibali said, adding: "At the time, I preferred not to mention it, but in 2010 Froome was excluded from the Giro for being towed by a car…"
Froome released limited power data during his Tour win and pledged to undergo physiological testing in the future [no date has been confirmed – ed.] in a bid to alleviate doubts over the legitimacy of his performances.
"It so happened that we gave important data from 2013 to students for their university thesis, and nowadays with the GPS on the bikes, you can obtain a lot of parameters," Nibali said. "But it's not right either to give a competitive advantage to your rivals."
Nibali's thoughts were echoed by Aru. "I agree," he said. "Especially when you're talking about training data that help people see where you are in your preparation and how you prepare for your targets."
It remains to be seen if Nibali and Aru will be on the same page at the Vuelta, and beyond the Spanish race, Astana have yet to decide how they will continue to divide their responsibilities next season, the last of Nibali’s current contract with the team.
"Fabio's developed quickly, he's very strong, but for another year at least, we'll be teammates," Nibali said. "Our programmes for 2016 haven't been decided yet. For instance, I'd like to go back to the Giro, but I don't know yet if that will be the case."
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