By his own admission, Froome "just didn't have the legs" on the last of four climbs at the end of the 214.5 km slog that took almost six hours to complete and which has thrown the general classification wide open with a week still to go. There are two likely sprint finishes left and a possible third, all of which favor Kittel, so Matthews would have to score serious points on stages 16 and 17, with Kittel drawing nearly none, to really have any shot at taking the lead.
"Froome is super", Frank said.
In fourth is Rigoberto Uran, who stuck with Froome on the final climb.
"Uran is more of the dark horse who slips under the radar but he is the strongest time-trialist of the group of riders (challenging me)", said Froome, who believes he timed his peak of form well.
It had been clear since midway through the day - a 189.5km stage through the lovely Massif Centra range to Le Puy en Velay - that stage honours would go to the breakaway, with Sky happy to let a 28-man group pull further and further away.
Landa is now up to fifth overall, just 1 minute 9 seconds behind Astana's Aru, who leads Froome by 6 seconds.
"Guys are exhausted and we have a super-strong team here and we have showed that today". He trails the Briton by 29 seconds overall. He thanked his teammates for their essential role in keeping him at the front, allowing him to pounce on the final climb while Aru was stuck.
The Australian even sprinted for 16th place, although points at the finish go down only to 15th but then, as Kittel said, this fight could last as far as the Champs-Élysées next Sunday. Eight players are just separated by 2:22 seconds with seven stages left.
"I knew he would try something", he said. Froome congratulated him, saying: "The race is certainly on now".
"I very rarely make a plan, it is just [about] opportunities", said Martin afterwards. He had a lead of 1:47 after Stage 14 in 2016, of 3:10 in 2015 and 2:28 in 2013.
Aru was the big victor in Peyragudes, where he took the yellow jersey for the first time after Froome cracked in the last 500 meters of the stage. "I could sit up and enjoy the win in the Tour de France".
Despite riding in support of Chris Froome, Landa has been impressive in mountain stages and was stronger than Froome last week in the Pyrenees when the Team Sky leader struggled in the ascent to Peyragudes.
Martin first made his name as a one-day classics specialist, winning Liege-Bastogne-Liege in 2013 and the Tour of Lombardy in 2014, meaning he's won two of cycling's five "Monument" one-day classics.
Froome suffered a mechanical problem 50km from the end of the 189.5km 15th stage from Laissac-Severac l'Eglise to Le Puy en Velay that saw him drop 50 seconds behind his main rivals at one point. The thrillingly tight grouping at the top promises vigorous battles between them in the last week of racing.