After attacking before the main climb of the day, the Col d'Aspin, the Brit extended his lead on the ascent maintaining an advantage over the chasers to take a memorable win. Orica-BikeExchange's Daryl Impey out sprinted Daniel Navarro (Cofidis) to claim second.
"That was sweet. After what has already happened this week, it is just fantastic to have won. Of all my victories, I think this has to be my best one," Cummings said.
"I wasn’t confident in that big group and putting pressure on them, I thought, was my best option.
"The group behind was obviously on the limit so I just carried on, as you do, and I was able to win."
In the main breakaway of the day, race leader Greg van Avermaet (BMC) battled all the way up the Col d'Aspin to hang on to his advantage on the general classification. His lead now stands at five minutes and 50 seconds over second-placed Adam Yates (Orica-BikeExchange).
"I knew it would be hard to keep the jersey today. I made a smart move I think to go in the break. It was a hard start and everybody wanted to go," Avermaet said.
"It kept the pressure off the team, they didn't have to work that hard today and I keep yellow. It was a great day, it's not often you see yellow in the breakaway. It wasn't easy.
"With yellow on the shoulders you always do a little bit more. I was really motivated to keep the jersey. I just wanted to go for it, the shape is really good and I'm happy to have another day."
How it happened
It was a tough battle from the start to stick an early breakaway, with a number of moves attempted.
A large group of 29 riders eventually formed and surprisingly included Avermaet and Giro d'Italia winner Vincenzo Nibali (Astana). Cooperation among the group was limited as they pushed out to a maximum advantage of five minutes and 30 seconds.
Back in the peloton, Sky and Movistar set the pace on the run into the Col d'Aspin, but were content to let the break fight it out for the stage win allowing the gap to hover at the five minute mark.
Nibali attacked the break with 40 kilometres to go, provoking a split of three off the front with Navarro, Matti Breschel (Cannondale-Drapac) and Antoine Duschesne (Direct Energie) swiftly gaining a decent advantage. The chasing group splintered behind as the result of counter-attacks and Steve Cummings bridged over to the front three.
The pursuers from the original move became more organised behind the lead group of four but just as they almost made contact, Cummings used a traffic island to shield a surprise attack to jump away from his rivals. He hit the bottom of the Col d'Aspin with just over 30 seconds advantage over the chasing group of Nibali, Impey, van Avermaet and Navarro and the peloton a further four minutes back.
The Englishman managed to extend his gap steadily, pushing it out to a minute despite Nibali leading the chasers up the ascent. Navarro and Impey attacked the Italian near the summit of the Col, trying to get back on terms with Cummings, but he descended sharply leaving time savour the win crossing the line with arms outstretched.
Back in the peloton
Warren Barguil (Giant-Alpecin) tried an exploratory move near the base of the Col d'Aspin, but was quickly shut down by FDJ riding as a team to bring him back. It was therefore a surprise their team leader Thibaut Pinot was the first big rider to crack, dropping off the back of the main bunch. Pinot looked in a lot of distress as he faded off the back, sweating profusely and looking very weak on the way to losing almost three minutes on his GC rivals.
Over the top of the climb, Daniel Martin (Etixx-QuickStep) attacked with Adam Yates (Orica-BikeExchange) in tow and the pair looked to have built up a small advantage on the other contenders before running into the collapsing inflatable one kilometre to go mark.
With Yates crashing severely and injuring his face, other riders lifted their bikes over in a frantic attempt to get through the banner. The chaotic scene saw the main bunch soft pedal into the finish with a truce between the big names after the confusion caused by the downed inflatable arch.
After the stage, Adam Yates was back in 30th and over 10 minutes behind on the official standings, but this was later overturned and he is now in second overall and takes the lead in the young riders classification.