The 21-year-old Spaniard held off a late charge from 16th seed Humbert to win 6-3 6-4 1-6 7-5 and move into the last eight at SW19.
It took Alcaraz’s winning streak at Grand Slams to 11 following his French Open triumph last month.
“I feel great playing today, I think I played a really high level,” he said.
“Playing a leftie, I try not to think about it and play my own game.”
Alcaraz will next face American 12th seed Tommy Paul, the Queen’s Club champion, who defeated Spain’s Roberto Bautista Agut 6-2 7-6 (7-3) 6-2.
Meanwhile, world number one Jannik Sinner defeated American 14th seed Ben Shelton 6-2 6-4 7-6 (11-9).
Shelton, who won in five sets in his first three rounds, had four set points in the third-set tie-break.
Sinner, 22, will play Daniil Medvedev next after the Russian fifth seed benefitted from Grigor Dimitrov’s early retirement with a leg injury.
The Bulgarian 10th seed took a medical timeout before deciding, at 5-3 down in the first set, not to continue.
It seemed unlikely Sunday’s opening match on Centre Court would live up to the five-set classic Alcaraz played against Frances Tiafoe on Friday.
And it became even more improbable when, with little effort, he found himself two sets up in what appeared a relatively dull encounter.
But the three-time major winner suddenly faltered in the third set and Humbert stepped up to take advantage.
The Frenchman, 26, raised his intensity, hitting the chalk with brilliant winners and breaking serve three times to force a fourth set.
Both players struggled to hold in a tense set, and an increasingly frustrated Alcaraz turned to the crowd and his team for help, waving his arms animatedly to stir up support.
With their backing, the defending champion found another gear, impressively holding his serve under immense pressure and breaking for a 6-5 lead. Alcaraz then served out the match and, relieved, he applauded all four corners of Centre Court in appreciation.
Under the roof on Court One, Shelton, perhaps feeling the effects of playing on a sixth day out of seven in the tournament so far, fell two sets down in just over an hour.
However, Shelton fought back in the third and used a boisterous Court One crowd to break for a 2-0 lead.
Australian Open champion Sinner pulled level and pulled off a stunning between-the-legs shot from a deep Shelton return that drew gasps from the crowd.
After saving the set points, Sinner’s victory was secured when Shelton double-faulted at 10-9 down.
“It was a tough match, especially the third set. I had to keep saving set point. These matches can go long but I was glad to close it in three,” said Sinner.
