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It was a gutsy display from the world No.3 from Spain, who was on the verge of defeat on more than one occasion, saving three break points at 5-5 in the second set and coming back from 1-3 down in the third to earn  a hard-fought victory.

"I just wanted to stay strong and wait for my chances," Alcaraz said.

"I think his level is high right now and he puts a lot of pressure on his opponents. Today I could feel it but in some moments he just made a few mistakes and I tried to make the most of those points and wait for my chances. The second set I saved those break points and then made the only break point I had that set.

"In the third set I was just trying to play good tennis. When I was a break down I just tried to keep going and play aggressively and be strong mentally and physically."

Alcaraz suffered a shock opening-round exit to David Goffin in Miami, but appears to be on the return to his best in The Principality, but this was a tougher than expected test against Fils, whom he had not played before on the ATP Tour.

In-form Alejandro Davidovich Fokina awaits Alcaraz in the semi-finals after he beat Australia's Alexei Popyrin in their last-eight clash.