Portugal's Rui Costa claimed his second win of the 100th Tour de France on Friday after soloing to victory in the 204.5km 19th stage.
Movistar rider Costa shot out of a chasing peloton on the fifth and final climb of the 204.5km ride from Bourg d'Oisans to Le Grand Bornand to catch stage leader, Europcar's Pierre Rolland.
The Portuguese, taking his second win and the third of his career following a victory in similar style on stage 16 to Gap, crossed the finish line 47secs ahead of German veteran Andreas Kloden (RadioShack).
"I'm really happy to have won but to be honest I didn't really believe in my chances at the start of the stage," said Costa.
"That's why I decided to wait until the final climb to make my move."
Team Sky leader Froome finished just under nine minutes in arrears but maintained his 5min 11sec overall lead over Spanish rival Alberto Contador.
"To be over five minutes ahead of the second place wearing the yellow jersey is just amazing," said Froome.
"It's going to be very hard for someone to take more than five minutes off me in 125 kilometres (on Saturday), but I don't want to be complacent. I have to stay switched on, at least until tomorrow evening."
On the penultimate day in the mountains there was ample opportunity for Contador's Saxo team to try to claw back their deficit.
But as an early breakaway forged ahead of the main bunch, the Spaniard's challenge failed to materialise.
Saxo's accelerations helped drop several of Froome's teammates on the penultimate climb, but the Kenyan-born Briton ultimately crossed the finish safely on the wheel of Australian teammate Richie Porte.
Contador virtually admitted his hopes of a third victory, after wins in 2007 and 2009, were over.
"We attacked a little and I spoke with my teammate Roman (Kreuziger) about the possibility of attacking," said Contador.
"I felt good, but I didn't have strong enough legs to really attack. In the end we thought it would be best to come over the finish together."
Contador's biggest worry may now be protecting his runner's up place from Colombian climbing specialist Nairo Quintana, who is third overall at 5:32.
Of the Australians, Michael Rogers dropped from eighth overall to 14th at 19:04 with Porte 20th at 37:53.
Ahead of Saturday's final day in the Alps, the 125km 20th stage from Annecy to the summit of Annecy-Semnoz, Froome looks virtually assured of succeeding compatriot and teammate Bradley Wiggins as the race champion.
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