Attacks by Froome's rivals Alberto Contador (Tinkoff-Saxo), Vincenzo Nibali (Astana) and Nairo Quintana (Movistar) all proved fruitless as the overall contenders finished in a group together in Saint-Jean-de-Maurienne.
Three minutes earlier, Romain Bardet (Ag2r) had taken a solo stage victory after attacking from the escape group. Fellow Frenchman Pierre Rolland (Europcar) finished second, with Colombian Winner Anacona (Movistar) in third.
The day's racing was shaped by a large, 29-rider escape group which included Bardet, Rolland, Joaquim Rodriguez (Katusha), Andrew Talansky (Cannondale-Garmin), Daniel Martin (Cannondale-Garmin), Thibaut Pinot (FDJ), Thomas Voeckler (Europcar) and Briton Simon Yates (Orica-GreenEdge).
Rodriguez had put himself into the escape to collect up as many mountains points as possible - and with seven categorised climbs along the route, he had plenty of pickings. Although he was wearing the polka-dot jersey of mountain leader, Rodriguez started the day in second place in that classification behind overall leader Froome. Having crested the day's opening five categorised climbs first, Rodriguez moved into the KoM lead - but is now tied on points with stage winner Bardet.
The escape split on the Col du Glandon, leaving a lead group of 11 riders - Yates not among them. Behind, the peloton also split as the gradient increased leaving the overall contenders pulled along by Team Sky.
Mathias Frank (IAM Cycling), Robert Gesink (LottoNL-Jumbo) and Warren Barguil (Giant-Alpecin) attacked the peloton on the Col du Glandon. They were joined by Contador, who broke free from the Froome group with 8km to go until the summit.
Nibali was the next to attack near the summit, followed by Quintana. Geraint Thomas paced Sky leader Froome back up to them with the result that Alejandro Valverde (Movistar) was temporarily distanced. Froome, Nibali, Quintana, Valverde and Contador all came back together on the descent.
Meanwhile, Bardet had gone solo from the remnants of the escape group, sweeping through the bends on the long downhill. He hit the bottom of the final climb of Lacets de Montvernier with 40 seconds advantage over the chasers and three minutes on the Froome group - a gap that he managed to maintain until the summit and on to the finish line.
Despite a last-ditch attempt to distance Froome over the top of Lacets de Montvernier, the contenders all crossed the finish line together with no change to the top order of the general classification and with two days in the Alps still to go.
Froome continues to lead Quintana by 3-10, with Valverde in third at 4-09. Thomas is fourth at 6-34, and Contador fifth at 6-40. The only change in the top 10 was Bardet moving up to 10th, swapping places with Barguil, now in 11th.
Mark Cavendish's key Etixx-QuickStep lead-out man Mark Renshaw was one of the day's most notable withdrawals, leaving the Manxman with a depleted set of team-mates for Sunday's sprint finale in Paris.
Also notable was that the stage was the first day since the opening time trial - excluding the stage nine team time trial when no points were awarded - that points classification leader Peter Sagan (Tinkoff-Saxo) had not scored a point.
Another big day in the Alps awaits the riders on Friday, with stage 19 running from Saint-Jean-de-Maurienne to La Toussuire – Les Sybelles featuring the climbs of Col du Chaussy, Col de la Croix de Fer, Col du Mollard and the finish on La Toussuire.
>>> Tour de France 2015 stage 19 full preview
Tour de France 2015, stage 18: Gap to Saint-Jean-de-Maurienne, 186.5km
1. Romain Bardet (Fra) Ag2r in 5-03-40
2. Pierre Rolland (Fra) Europcar at 33 secs
3. Winner Anacona (Col) Movistar at 59 secs
4. Bob Jungels (Lux) Trek Factory Racing
5. Jakob Fuglsang (Den) Astana at same time
6. Serge Pauwels (Bel) MTN-Qhubeka at 1-01
7. Cyril Gautier (Fra) Europcar at 1-50
8. Damiano Caruso (Ita) BMC Racing at same time
9. Andrew Talansky (USA) Cannondale-Garmin at 1-55
10. Warren Barguil (Fra) Giant-Alpecin at 3-02
12. Chris Froome (GBr) Team Sky at 3-02
14. Geraint Thomas (GBr) Team Sky
15. Alberto Contador (Spa) Tinkoff-Saxo
17. Nairo Quintana (Col) Movistar
18. Vincenzo Nibali (Ita) Astana all same time
Overall classification after stage 18
1. Chris Froome (GBr) Team Sky at 74-13-31
2. Nairo Quintana (Col) Movistar at 3-10
3. Alejandro Valverde (Spa) Movistar at 4-09
4. Geraint Thomas (GBr) Team Sky at 6-34
5. Alberto Contador (Spa) Tinkoff-Saxo at 6-40
6. Robert Gesink (Ned) LottoNL-Jumbo at 7-39
7. Vincenzo Nibali (Ita) Astana at 8-04
8. Mathias Frank (Swi) IAM Cycling at 8-47
9. Bauke Mollema (Ned) Trek Factory Racing at 12-06
10. Romain Bardet (Fra) Ag2r at 12-52
Video: Key climbs of the Tour de France - Montvernier
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