On an odd night in Acapulco, where the longest three matches in the ATP tournament's history were played back-to-back-to-back, the longest of the bunch featured a heartening moment of sportsmanship.

Having won the first set against Grigor Dimitrov in an 18-point tie-break, Stefan Kozlov managed to break the world number 33 in the eighth game of the second, but there was no time to celebrate.

Immediately after Dimitrov's backhand hit the net, his American opponent suddenly started walking like a rusty tin man as cramp took hold of the lower half of his body as the match entered its third hour.

Dimitrov ran around to his opponent to check on him, trying to stretch Kozlov's legs as the trainers arrived.

No surprise then that Dimitrov rattled off the next four games to win the second set, but Kozlov regained his composure and perhaps a bit of blood flow to win the third set 6-3, taking the match in three hours and 21 minutes, making it the longest in the history of the Mexican Open.

It broke the record set minutes earlier on the same Acapulco court, when marathon match veterans John Isner and Fernando Verdasco spent three hours and 13 minutes on court, but that was pushed back to the third-longest match in tournament history when Alexander Zverev and Jenson Brooksby played for 3:20, finishing up at around 5am local time.

Making Kozlov's experience even more surreal was the fact he was not even supposed to be playing the match.

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He said he was practising with Rafael Nadal "across the street" when Nadal broke a string and Kozlov just happened to check his phone to find out Maxime Cressy had withdrawn from the match against Dimitrov.

"I just had an intuition and I have 20 text messages… and they're like 'get over here,'" he said.

"So I drop all my stuff, I tell Rafa, he's got a big smile on his face. He's super excited for me because he knew I was the number one lucky loser.

"Didn't help to be moved side to side by Rafa before the match."