The seasoned Argentine recovered to win 3-6, 7-6 (5), 6-4 in one hour, 55 minutes against the 20-year-old Zverev and will next play either Viktor Troicki or John Isner.
Zverev was going after his sixth title of an outstanding season and was comfortable in the first set against Del Potro, the 2009 US Open champion and 16th seed.
The German prodigy and the 29-year-old, whose career has been beset by injury, went toe to toe in the second set and the tie break was just as tight until Del Potro grabbed the crucial mini-break.
And then came the turning point – and the flash of anger from third seed Zverev – as he yielded the crucial service break to go 3-2 down in the third set.
“Right now I’m very pissed off, as you can probably tell,” said Zverev afterwards, having smashed the racquet on the court, then trodden on it, and brutalised it further as he stomped back to his chair.
A fan at least came away with a demolished racquet after Zverev handed it to a spectator.
“I don’t understand how I can lose a match where I get broken one time in three sets and not managed to get any chances on his serve,” said Zverev, the world number four tipped to one day top the rankings.
“I’m a bit pissed off because I feel like I played well and that’s the upsetting part of it – I could have done great in this tournament.”
The Bulgarian Dimitrov defeated American Sam Querrey 6-3, 7-6 (7/3) and Spain’s Ramos-Vinolas saw off Germany’s Jan-Lennard Struff 7-6 (7/4), 6-4.
— Live Tennis (@livetennis) October 12, 2017
Fognini was suspended from further participation in last month’s US Open after the incident, which meant he couldn’t compete in doubles, and was fined $24,000 for on-site offences.
On Wednesday, the Grand Slam Board announced further punishment for the Italian world No. 28, following an investigation into the matter.
“The Grand Slam Board Director determined that Mr Fognini had committed the Grand Slam Major Offences of Aggravated Behaviour and Conduct Contrary to the Integrity of the Game for his misconduct at the 2017 US Open,” read a statement released by the board.
The decision stated the following sanctions:
1. Mr Fognini is guilty of the Major Offence of Aggravated Behaviour (Article IV.A) and Conduct Contrary to the Integrity of the Game (Article IV.B);
2. Mr Fognini is hereby fined $96,000 (in addition to the $24,000 penalty already paid for on-site offences) which will be reduced to $48,000 if no further Grand Slam Major Offence conviction occurs through and including 2019;
3. Mr Fognini will be suspended from participation in two Grand Slam tournaments, one of which must be the US Open, except that any such suspensions will not be imposed if no further Grand Slam Major Offence conviction occurs through and including 2019;
4. If a further Grand Slam Major Offence is committed during the period, the full penalties will be re-instated in addition to any other penalties imposed for the subsequent Grand Slam Major Offence;
5. Mr Fognini accepts and will not appeal this Decision and he has expressed remorse for his admitted misconduct. He has confidence that he will meet the conditions necessary to reduce his financial penalty and to lift his Grand Slam tournament suspensions in the future.
6. As always, all financial penalties arising from misconduct at Grand Slam tournaments are donated by the Grand Slam tournaments to the Grand Slam Development Fund.
Fognini had made $1,177,715 in prize money so far this season (as of last Monday) and claimed his fifth ATP title in Gstaad in July. He reached the St. Petersburg final last month, where he lost to Damir Dzumhur.
The 30-year-old is no stranger to controversy, with his on-court temperament frequently landing him in hot water.
Temperamental Australian Nick Kyrgios was fined $10,000 and lost another $21,085 in prize money on Wednesday after storming off midway through his first-round match at the Shanghai Masters.
World number one Rafael Nadal meanwhile raced into the third round in just 54 minutes as he attempts to win one of the rare tournaments that has eluded him.
But it was the 22-year-old Kyrgios who was again in the spotlight following what was just his latest controversy on Tuesday.
The ATP said it had fined Kyrgios for unsportsmanlike conduct and he will not collect his prize money because he failed to get signed off for a medical after retiring from his match.
Kyrgios’s time at the Shanghai Masters is over after he also pulled out of his scheduled doubles match with Frenchman Lucas Pouille on Wednesday.
He apologised after walking off court on Tuesday, having lost the first-set tie break to the American Steve Johnson.
In a statement posted on Twitter a few hours later he said that he had a “stomach bug” and had hurt his shoulder. The tweet later appeared to have been deleted.
It was a nasty case of deja vu for the world number 21 — a year ago he argued with spectators and was accused of giving away points in a defeat at the same Shanghai event.
The talented but fiery Kyrgios was consequently fined and suspended.
His immediate future in tennis is again under threat following Tuesday’s incident, when he hastily exited the court with jeers and boos ringing in his ears.
It came after he complained about the umpire and spectators in the arena, and was docked a point penalty for the second match running because of his conduct, which included blasting a ball out of the court in a flash of rage.
“What’s happening here?” he asked, seemingly irritated by spectators moving around between points in the tie break.
“Exactly why I shouldn’t come here,” he muttered.
Kyrgios appeared to blame the umpire for failing to get the spectators in line and his afternoon worsened fast when he received a point penalty.
“Poor officiating again,” Kyrgios complained, and added: “Sick of this place.”
A dominating first set from @RafaelNadal thanks to shots like this….
— Tennis TV (@TennisTV) October 11, 2017
If Nadal was fast in dumping out the American Jared Donaldson, rising star Alexander Zverev was even faster. He was on court for just 12 minutes when Britain’s Aljaz Bedene retired hurt.
The unseeded Bedene was losing 4-0 in the first set when he called it quits because of a leg injury.
The 20-year-old Zverev, the German prodigy who is seeking his sixth title of a breakthrough season, joins sixth seed Grigor Dimitrov in the third round.
But the Bulgarian had an almighty scare and saved three match points before pulling through 3-6, 6-3, 7-6 (6) against the American Ryan Harrison.
Also through in Shanghai, where matches were forced indoors because of the rain, were Gilles Simon of France and Sam Querrey of the United States.