Froome crossed the line in third place ahead of BMC’s Richie Porte, but with Thomas down in tenth place the Welshman surrendered the race lead he took in Saturday’s opening time trial in Dusseldorf.
Three-time Tour winner Froome now leads by 12 seconds from Thomas, with Aru up to third, 14 seconds back, after the Italian national champion won his first career Tour de France stage.
Froome said: “I remember the first time I was in yellow in 2013. It was a bit of an overwhelming experience to be honest, but I think I’ve spent enough days in yellow now to get used to that. I know what I’m up against. This is going to be the hardest-fought battle in terms of the general classification and I know my rivals are right up there.”
The 160.5 kilometre stage from Vittel was billed as the one which would bring true shape to the fight for yellow as the first of only three summit finishes in this Tour, and it delivered. This is only the third time the Tour has visited La Planche des Belles Filles – scene of Froome’s first career Tour stage win in 2012. But it is worth noting that on both previous occasions the man wearing yellow at the end of the day – Sir Bradley Wiggins in 2012 and Vincenzo Nibali in 2014 – was wearing it in Paris at the end of the race.
The strength of Aru’s attack will certainly have caught the attention of his fellow contenders as the 27-year-old burst clear with 2.4km left and won by 16 seconds from Martin, with Froome leading Porte over the line another four seconds back.
Martin said: “It was a cat and mouse game when Fabio went. We kind of looked at each other. He was super strong so congratulations to him. I knew I just had to wait for a sprint and if Fabio was close enough I’ll be able to catch him but in the end it didn’t work out.”
Romain Bardet, France’s great hope for a home win, was fifth on the day, just ahead of Britain’s Simon Yates and former Team Sky man Rigoberto Uran and two-time Tour winner Alberto Contador. But Nairo Quintana could only manage ninth place, 34 seconds behind Aru, as his hopes of contending suffered another blow.
Martin is up to fourth place overall, 25 seconds back. The Quick-Step Floors man is 14 seconds ahead of Porte and 18 ahead of Orica-Scott’s Yates, who moves up to sixth overall and takes over in the white jersey as the best young rider – a category won by his twin brother Adam 12 months ago.
Quintana is already in trouble, some 54 seconds behind Froome after five stages.
With Aru now only 14 seconds off yellow, the Italian will have seized leadership of the Astana team as Jakob Fuglsang, billed as a co-captain, faded badly on the imposing gradients late on this relatively short but extremely sharp climb.
Nobody responded when Aru launched his move just before the gradients – which average 8.5 per cent over the course of the 5.9km climb – began to ease off slightly, and the Italian was quickly out of reach. “I made a little bit of a mistake at the final by giving Aru that much space to go,” Froome said. “But with it being flat on the final kick, no-one wanted to be left pulling on that final point.”
The others put in little digs at one another but it was not until the final few hundred metres, where the road ramped up to 20 per cent, that Martin burst clear and Froome rounded Porte to claim the last of the bonus seconds on offer.