Guess what? It happened.
Siemerink claimed the next eight points and McEnroe did as he promised in a later match at the season's final major.
No commentators were believed to have said something similar when Stan Wawrinka trailed Guillermo Garcia-Lopez 5-0 and 6-2 in a fourth-set tiebreak Monday at the Australian Open -- and lucky for them.
Just like Siemerink, Wawrinka did the almost unthinkable and pulled off a great escape.
The defending champion at the year's opening grand slam saved five set points in the tiebreak to close out the encounter against the Spaniard 7-6 (2) 6-4 4-6 7-6 (8) and advance to the quarterfinals.
"Losing 5-0 was a bad start to the tiebreak," Wawrinka, upset by Garcia-Lopez at the 2014 French Open, told reporters. "At 6-2 I knew it was close to come back because I had the wind with me.
"I had to focus on every point. I knew if I was going to come back (to) 6-5, (he) was going to get nervous. Just focused point after point."
Wawrinka is flying the Swiss flag after Roger Federer's surprising loss to Andreas Seppi in the third round and the world No. 4 will have to maintain, or increase, his level if he is to continue his winning streak at Melbourne Park.
Wawrinka's quarterfinal opponent, Kei Nishikori, beat the 29-year-old in the last eight in September en route to making the final at the U.S. Open.
"I'm happy the way I'm playing so far," Wawrinka said. "I'm ready for the tournament. My game is there. I didn't spend too much energy on the court since one week.
"I'm happy to get the chance to play one more quarterfinal."
Nishikori did something rare later Monday on Australia Day when he downed David Ferrer 6-3 6-3 6-3.
Only Rafael Nadal and Novak Djokovic had beaten the Spaniard -- one of the game's top competitors -- in straight sets at a hard-court or clay-court major since the beginning of 2011.
It was also the first time Nishikori defeated Ferrer in straight sets, his previous six victories all going the distance.
World No. 1 Djokovic, going for a fifth Australian Open title, will take on Canada's Milos Raonic in the quarterfinals. Djokovic overcame Gilles Muller in straight sets.
Raonic, meanwhile, eliminated Feliciano Lopez in five sets.
Lopez saved match points in two encounters earlier in the fortnight and saved another versus Raonic in the fourth set. But Raonic took the fifth 6-3 to prevent a rare hat-trick.
Another early wobble for Serena
For the third straight match, women's top-seed Serena Williams survived an early wobble to progress.
She saved three set points in the first set against Vera Zvonareva, lost the first set against Elina Svitolina and then lost the first set to Garbine Muguruza -- her conqueror at the French Open -- on Monday.
But Williams upped her game despite dealing with a cough and got a bit of luck in the third set to oust the big, flat-hitting 24th-seed 2-6 6-3 6-2 in the fourth round.
"I've been playing for a long time," Williams, bidding for a 19th major but first in Australia in five years, told reporters. "When I have to go up a level, I have to.
"I can't afford to stay at the same level or I will be where I was at the French Open. I definitely didn't want to be there without at least trying to give 1000%."
Still the match might have turned out differently had Muguruza not missed a routine volley on break point when leading 1-0 in the third.
"I knew that if I won this game, I would have more chances to win the third set, because (to) break her serve, it's difficult," said Muguruza. "I wanted to use that opportunity, but I couldn't."
Williams returned to the Rod Laver Arena later on Monday, rooting on older sister Venus.
And she left in good spirits, since Venus Williams upset sixth-seed Agnieszka Radwanska 6-3 2-6 6-1 to make a first grand slam quarterfinal in five years. Venus Williams, too, hadn't beaten a top-10 player at a major since the 2010 U.S. Open.
"She really had answers for everything today," said Radwanska.
When Victoria Azarenka upset Caroline Wozniacki in the second round, most would have expected Serena Williams to face the two-time Australian Open winner in the quarterfinals.
But Dominika Cibulkova didn't read the script.
The 2013 finalist defeated Azarenka 6-2 3-6 6-3.
Madison Keys -- who sent Wimbledon champion Petra Kvitova packing -- reached her first grand slam quarterfinal by beating compatriot Madison Brengle 6-2 6-4.