Like Zverev, the 20-year-old Fritz is a former junior world number one ear-marked for great things but yet to truly arrive at the highest level.
"I don't know how far I will go in the tournament but I'm confident", Del Potro said.
Greek teenager Stefanos Tsitsipas struck 25 winners in a clinical display to beat Italian Thomas Fabbiano 6-2 6-1 6-4.
Fourth seed Alexander Zverev reached the third round at Wimbledon for the third year in succession when he came from 2-1 down to complete a 6-4 5-7 6-7 (0) 6-1 6-2 victory over Taylor Fritz in their suspended match on Friday.
Venus, seeded ninth, was beaten 6-2 6-7 (5) 8-6 by Dutchwoman Kiki Bertens on Court One.
"Today, I was returning much better, as I said, because it was brighter". "I think he's going to be able to do it". "I feel 100 percent comfortable with contacts".
"For me, we played in darkness for one and a half hours. I don't think I did a good job of keeping in the moment and playing the person who was in front of me". I'll do the best I can to prepare myself for the hard courts now'.
"That's why I couldn't really see his serve, I couldn't really break him". I asked the chair umpire how is it possible he plays that shot if it didn't bounce twice.
His only quarter-final result at the Grand Slams came at this year's French Open, where he lost 6-4, 6-2, 6-1 to Austria's Dominic Thiem.
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"What can I do?"
Next up for Djokovic will be 22-year-old Russian Karen Khachanov, who rallied from two sets down to defeat American Francis Tiafoe 4-6, 4-6, 7-6 (7-3), 6-2, 6-1 on Saturday.
"I'd rather go five than lose in four, be out of here, book a flight for me to go home tomorrow".
Gulbis, a former World No. 10 who famously defeated Roger Federer on his way to the French Open semifinals in 2014, will be playing in the last 16 at Wimbledon for the first time at the 11th attempt.
His win on Saturday was his third successive five-setter having also played three matches in qualifying.
Switzerland's Belinda Bencic, a former top 10 player whose ranking slumped out of the top 300 previous year after wrist surgery, made the fourth round for the second time with a 6-1, 7-6 (7/3) win over Spain's Carla Suarez Navarro.
The German next faces Latvian qualifier Ernests Gulbis for a place in the last 16.
He added: "Yesterday I had pain during the whole three sets".
"I'm very happy (to make the last 16 for the first time), long time I've been chasing for that", said Monfils, who will next face South African Kevin Anderson.