Novak Djokovic’s US Open quarter-final victory over John Millman was momentarily held up by one of the most bizarre events seen at a Grand Slam as the Australian left midway through the second set because he was sweating too much and soaking the court surface in the process.

The 29-year-old already had the unenviable task of taking on 13-time Grand Slam champions Djokovic, but his task was compounded by the match taking place in searing temperatures and humid conditions that plagued this year’s tournament.

Millman left the court when the scores were 2-2 in the second set to change his attire, with the chair umpire allowing him to leave without punishment as it met the United States Tennis Association’s ‘Equipment Out of Adjustment’ regulation.

US Open day 10 Show all 16 1 /16 US Open day 10 US Open day 10 Novak Djokovic vs John Millman The heat once again took its toll on players at the US Open. Getty US Open day 10 Novak Djokovic vs John Millman Novak Djokovic compared the Arthur Ashe Stadium to a sauna/ EPA US Open day 10 Novak Djokovic vs John Millman John Millman was unable to repeat his heroics in beating Roger Federer as he slipped to defeat. Getty US Open day 10 Novak Djokovic vs John Millman Djokovic managed to win the quarter-final in straight-sets. Getty US Open day 10 Novak Djokovic vs John Millman Djokovic celebrates his victory over Millman. Reuters US Open day 10 Novak Djokovic vs John Millman Millman congratulates Djokovic on his victory. EPA US Open day 10 Kei Nishikori vs Marin Cilic Kei Nishikori celebrates after defeating Marin Cilic. AFP/Getty US Open day 10 Kei Nishikori vs Marin Cilic The pair shake hands after an epic five-set quarter-final. Reuters US Open day 10 Naomi Osaka vs Lesia Tsurenko Naomi Osaka joined compatriot Nishikori in the semi-finals. Reuters US Open day 10 Naomi Osaka vs Lesia Tsurenko Lesia Tsurenka crumbled as Osaka won in under an hour. AFP/Getty US Open day 10 Naomi Osaka vs Lesia Tsurenko The pair shake hands after the quarter-final. AFP/Getty US Open day 10 Naomi Osaka and Kei Nishikori Naomi Osaka and Kei Nishikori both won in the first time two Japanese players reached the semi-finals of a Grand Slam. Getty US Open day 10 Madison Keys vs Carla Suarez Navarro Madison Keys maintained home hopes with a victory over Carla Suarez Navarro. Getty US Open day 10 Madison Keys vs Carla Suarez Navarro Keys slipped during a point the resulted in her doing the splits. EPA US Open day 10 Madison Keys vs Carla Suarez Navarro But she recovered to secure victory over the Spaniard. Getty US Open day 10 Madison Keys vs Carla Suarez Navarro The two shake hands after their quarter-final. EPA

The USTA said afterwards that Millman spoke to the chair umpire to say that he was sweating excessively and that the court was becoming slippery as a result, which ultimately made conditions dangerous to compete in.

Before leaving, Millman also spoke to Djokovic to apologise for the delay, saying “I’m sorry, man, I’m going to have to change,” though the Serbian was more than thankful for the break as he sat in his chair shirtless in an effort to cool down.

“I was struggling. He was struggling,” Djokovic said. “He was apologising he had to go change. I said, ‘Man go ahead. I’m ok just sitting down and relaxing.’ I needed that rest. It was great.”

Millman was sent to change his shirt by the chair umpire (EPA)

The USTA confirmed that the unusual events met their regulations that allowed for a break in play midway through the match. A statement read: “At two games all in the second set of the Novak Djokovic-John Millman match, Millman approached the chair umpire to note his excessive sweating and the moisture it was leaving on the court. The chair determined that the surface was dangerous enough to invoke the ‘Equipment out of Adjustment’ provision in the ITF Duties and Procedures for Officials and allowed Millman to go off court to change clothes/shoes.

“Both players agreed that he should do so. Because the chair umpire deemed the situation within the ‘Equipment out of Adjustment’ provision, Millman was not charged with an official change of attire or bathroom break.”

Novak Djokovic compared the Arthur Ashe Stadium to a sauna (EPA)

But that was not the only strange activity during the last-eight encounter. Moments before Millman left, Djokovic was frantically searching through his bag and was heard to be looking for “tablets”, before saying to the umpire that he needed “the tablets that my wife brought from the apartment”.

While Millman was getting changed, a bottle that contained the tablets was delivered to the 31-year-old, but the story dragged on after the match as Djokovic refused to say what the tablets were. “I can't talk about that,” he said. “They arrived in the end.”

Djokovic went on to secure a 6-3, 6-4, 6-4 victory to progress to the semi-finals at Flushing Meadows, where he will face Kei Nishikori on Friday after the Japanese beat Marin Cilic in the quarter-finals.

But Djokovic remained critical of the conditions after the match and agreed with Roger Federer that the roof on the Arthur Ashe Stadium, which was completed in 2016, has had an impact on air ventilation to keep the players cool.

“Whether it's night or day, we just don't have air down there,” he said. “It feels like a sauna.

“I personally have never sweated as much as I have here. Incredible. I have to take at least 10 shirts for every match. After two games you're literally soaking. Roger never sweats that much, but look how he was a couple of nights ago.”

The heat once again took its toll on players at the US Open (Getty)

The criticism was added to by Rafael Nadal, who following his five-set victory over Dominci Thiem on Tuesday took a shot at organisers for the scheduling of matches at the US Open. The quarter-final finally reached a climax at 2.04am in the early hours of Wednesday morning, four hours and 49 minutes after it started, and the world No 1 questioned why they are being made to play so late in the night.