The match on Court 18 was halted at around 9.10pm yesterday for bad light and will continue no earlier than 3.30pm today. It has already surpassed ten hours in length, with the final set clocked at over seven hours.
Mahut saved two match points at 33-32 and another at 59-58, shortly before the game was paused. Isner had won the first set 6-4, with Mahut winning the next two 6-3 and 7-6, before Isner drew level with a 7-6 in the fourth. Neither man has broken the other's serve in the fifth set.
The previous record for the longest ever match was set in 2004 by Fabrice Santoro and Arnaud Clement, with the former winning after six hours and 35 minutes. Meanwhile, Isner has fired 98 aces to Mahut's 95, both smashing Ivan Karlovic's 2009 record of 78.
Mahut told BBC News: "We're fighting like we have never done before. We'll come back tomorrow and see who is going to win this match. Everyone wants to see the end."
Isner added: "He's serving fantastic, I'm serving fantastic. Nothing like this will ever happen again."
The winner will go on to face Thiemo De Bakker, who won his own final set battle 16-14 against Santiago Giraldo.