Andy Murray has admitted that he did not know what happened to his fitness in his first round win over Robin Haase at the US Open on Monday.
The eighth seed was in danger of losing the match from two sets up after suffering from cramp to his thigh, as he lost nine straight games, before fighting back from a break of serve down to win the fourth set and complete a 6-3, 7-6, 1-6, 7-5 victory.
The cramp suffered by the Briton came as a mystery, particularly as he has been putting in a high amount of training with coach Amelie Mauresmo over the summer, and he has not played an extensive amount of tennis in the lead up to the tournament, having been knocked out in the quarter-finals of Masters Series events in Toronto and Cincinnati.
After the match, the 27-year-old seemed as confused as anyone over what was behind his lack of mobility, and resilience saw him get through the match.
The two-time Grand Slam winner told reporters: “I started cramping at the beginning of the third set, at the front of my quad and then in my forearm.
"It was not particularly comfortable and I just tried to hang around as long as possible and managed to get through.
"I was in a good position at 2-0 up but I didn't know whether to go for it at the beginning of the third set or conserve energy and go for it in the fourth.
"I need to work out why that happened today because it shouldn't happen after an hour and 45 minutes, even though it was hot."
It was a competitive start to the tournament at Flushing Meadows for the 2012 champion, as he almost squandered a 4-0 lead and double break in the opening set, as his Dutch opponent fought back to 5-3 before failing to convert a break point when Murray served for the set, and the Dunblane-born player managed to serve it out.
An exchange of breaks early in the second set followed, and Murray once again came close to fluffing his lines, as he failed to convert his first two set points in the tie-break, before winning it 8-6.
Change in momentum
But the tide began to turn in the third set, as Murray was struggling to make any movement towards the ball, and he lost six straight games from 1-0 up.
With the high temperature adding to his woes, Murray asked the umpire if ice could be brought onto the court, which he placed on to his left thigh.
Things were looking ominous for the former world number two when he had his serve broken for the sixth time in the match at the start of the fourth set, but a double fault allowed him to break back, and he broke again in the 11th game before serving it out.
Next up for Murray will be qualifier Mattias Bachinger of Germany in the second round on Wednesday, who caused an upset by beating Czech Radek Stepanek 6-3, 6-3, 6-2.