Team Sky rider Froome leads Spaniard Contador by five minutes 11 seconds with two stages left, one being Sunday's parade to the Champs-Elysées.
"It would take a lot for someone to take more than five minutes," Froome told reporters after the stage which was won by Portugal's Rui Costa.
"Having said that, I don't want to be complacent so we have to stay focused and switched on until tomorrow night at least."
As Contador, who earlier this week had marked the stage featuring four demanding climbs as the one where he could overhaul Froome, settled for second place, his Saxo-Tinkoff outfit rode to defend top position in the team standings.
"That was not our objective at the start of the Tour but at some point you have to admit that Chris was just much stronger than anyone else in the race," Saxo-Tinkoff sports director Philippe Mauduit told reporters.
Contador will also need to defend his second overall as only 47 seconds separates him from fifth-placed Joaquim Rodriguez going into Saturday's 20th stage, a 125-km ride from Annecy featuring six climbs with a summit finish at Semnoz.
Spaniard Rodriguez and Colombian climber Nairo Quintana, who holds the white jersey for the best young rider, finished Friday's stage with Froome and Contador and pose the biggest threat.
They were, however, controlled by Contador's team all day.
"Maybe they were preventing attacks from Movistar (Quintana's team), it was perfect for us," Team Sky sports director Nicolas Portal told reporters.
COSTA CELEBRATING
Costa attacked on the final climb of the stage from Bourg d'Oisans and the Movistar rider pulled far enough clear of his rivals to start celebrating with a kilometre to go.
Germany's Andreas Kloeden was next over the line, 48 seconds behind Costa and Jan Bakelants of Belgium was third after the peloton rode through thunderstorms in the finale.
It was Costa's second stage win after the Portuguese had taken another solo victory in Gap on Tuesday.
"I'm in great form, I knew I had to be in the breakaway but I did not have victory in mind," he told a news conference.
Last year's Giro d'Italia winner Ryder Hesjedal of Canada went solo from an early breakaway group before being joined by France's Pierre Rolland five kilometres from the top of the Col de la Madeleine.
It proved too much of an effort for the Garmin-Sharp rider, who let Rolland ride away as the sky darkened with threatening clouds.
Just as the rain started to pour, Costa picked up Rolland and left him behind, riding solo with the lights of the motorbike brightening up his pedalling legs.
French champion Arthur Vichot crashed heavily in the descent but got back on his bike while German Marcus Sieberg left the race after also hitting the tarmac.
The 18 previous days of racing took their toll as Kris Boeckmans and Christophe Le Mevel pulled out midway through the stage.
(Editing by Alison Wildey)
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