Former champion Andy Murray was at a loss to explain an unusual bout of body cramp that nearly cost him a place in the second round of the US Open.
The eighth-seeded Scot could barely walk at times in the third and fourth set of his match on Monday (NZT early Tuesday) against Dutchman Robin Haase, but somehow survived to claim a 6-3 7-6 (6) 1-6 7-5 victory in steamy conditions at Flushing Meadows.
''I don't know exactly what it was, the heat or what, but I was cramping after an hour and 45 minutes, which isn't normal,'' Murray told Sky Sports in an on-court interview after rallying from 5-3 down in the fourth set.
''I didn't know what to do in that third set, my quads were cramping, then it started to get to my lats, then my forearms.
''I didn't know whether to conserve energy or try to finish it in three sets but then I got broken.
''I just tried to hang around and at the end I was trying to play without moving my legs much and managed to get through.''
Murray, renowned as one of the fittest players on the ATP Tour, arrived in New York raving about his mid-season fitness programme in Florida with new coach Amelie Mauresmo.
In the first two sets, though his tennis was patchy, Murray moved superbly well and his defensive skills gave him a commanding lead against world No 71 Haase, who took him to five sets when they met on the same court in 2011.
But from 2-1 in the third set, it was soon obvious that the 2012 US Open champion was in trouble as he stretched his right leg and had trouble pushing up on serve.
With Murray barely moving at times, Haase took advantage to take the third set and at 5-3, he served for the fourth only to falter.
Moving slightly better but still nowhere near 100 percent, Murray sliced and diced his way back to 5-5 and after an enormous forehand return winner gave him another break, he held serve to clinch a dramatic victory.
''These (grand) slams are physically challenging, but I need to work out why that happened because I shouldn't be cramping after one hour, 45 minutes, regardless of the temperature,'' said Murray, who will next play Germany's Matthias Bachinger.
''It was hot but it wasn't particularly humid and we didn't play a lot of long rallies.''