London, July 8 (IANS): Defending champion Carlos Alcaraz held off a late challenge from Frenchman Ugo Humbert to reach the men's singles quarterfinals at the Wimbledon Championships here on Sunday.

Alcaraz, 21, took the opening two sets 6-3, 6-4 in style, but struggled in the third set as he was broken three times by a fighting opponent, losing 6-1.

In the fourth set, both Alcaraz and the 26-year-old Humbert had their ups and downs, but the three-time Grand Slam champion from Spain played better on a few key points to come through 7-5, reports Xinhua.

"I felt like he was reading my serve and my shots very well in every point that we were playing. It was difficult for me to find the solutions in that moment," Alcaraz explained his struggle in the third set. "But tennis is like this. In the fourth set, there are a few serves that I did pretty well, and I saved that game. It was kind of I raised the level of my tennis, increased my intensity, and got the win in the end," he added.

World No. 1 Jannik Sinner dominated his match against American Ben Shelton, as the Italian pushed himself forward with a 6-2, 6-4, 7-6(9) victory in two hours and nine minutes.

Russia's fifth seed Daniil Medvedev won the fourth round as Bulgarian veteran Grigor Dimitrov retired while trailing 5-3 in the first set.

Women's singles 12th seed Madison Keys also retired midway due to injury in her match against Jasmine Paolini of Italy.

The Italian seventh seed will fight for a semifinal berth against American 19th seed Emma Navarro, who saw off compatriot and U.S. Open champion Coco Gauff 6-4, 6-3.

New Zealand qualifier Lulu Sun continued her miraculous journey, stunning former U.S. Open champion Emma Raducanu of Britain 6-2, 5-7, 6-2. Sun, 23, ousted China's eighth seed Zheng Qinwen in the first round, which was her first Grand Slam main draw victory.

"Playing all these players up until now, it's just a new opportunity each time. I'm super grateful to be able to play another match," said Sun.