Both sides then created few chances to take one point apiece following the thrilling four-goal draw.
"Now, I participate in eight to 10 races a year. I ran one triathlon and three marathons in a month last year," said Gu, who hopes to compete in the next Boston Marathon.
"I rode my tempo and gave it all to create a gap big enough to win."
"I don't think I have ever gotten defaulted yet in my career or in my life. No, I haven't been in a situation like that. But as I said, it's very unlucky, very unfortunate," Zverev added.
SIPG remain unbeaten with six wins and two draws before the match. They kept offense flowing and had a point-blank chance on 34 minutes, while captain Hulk blasted a close-range shot that was saved by China Fortune keeper Bao Yaxiong.
The 33-year-old striker, who came on after the interval, latched onto a through pass from teammate Wu Xi before sending it into the net in the 65th minute.
Djokovic's exit means there will be a first-time Grand Slam champion in the men's draw.
The remaining of the match have been controversial as Gao Lin was shown his fourth yellow card of the season in the 71st minute after complaining to the referee. The Chinese international is going to miss Shenzhen's next match against his former club Guangzhou Evergrande next Wednesday. Enditem
Shen's mother said she was happy to see her boy, who frequently got sick before, build up strength after taking up the sport, and added that she was considering letting Shen's eight-year-old sister have a try when canoeing classes return after the pandemic.
Elsewhere, fifth seed Alexander Zverev of Germany beat Frenchman Adrian Mannarino 6-7 (4), 6-4, 6-2, 6-2 to advance, while fourth seed Stefanos Tsitsipas of Greece was upset by Croatia's Borna Coric in a five-set thriller.
Looking at the transfer market, it is likely to be the only remodelling carried out by Real Madrid.