NEW YORK (AP) -- Steve Johnson's hand was cramping so badly he couldn't even hold his racket.

The American had to retire from his first-round match at the U.S. Open on Wednesday when his muscles suddenly seized up. The two-time NCAA champ led two sets to one and was up a break in the fourth before he collapsed on the court and started writhing in pain.

Earlier in the game, he landed awkwardly on his left ankle.

''The next point, my right calf just went kind of out of nowhere,'' Johnson said. ''Then it all kind of started to go.''

As medical staff attended to him, opponent Tatsuma Ito was, by rule, awarded point and game penalties because of the delay to go up 2-1 in the set. That brought on a changeover, and trainers massaged Johnson's legs while he drank fluids.

Johnson returned to the court but could barely move, forced to lightly tap his serves as if he was playing a recreational match. His arm also started cramping, and Ito, a 126th-ranked qualifier from Japan, went up 4-1 in the set.

During the next changeover, Johnson was in so much agony that he slid from his chair to the court and chomped on a towel. He then told the chair umpire he couldn't continue.

''That's the first time I felt pretty useless on the tennis court,'' Johnson said. ''I'm a competitive guy. I'll lay it on the line as much as I can.''

A wheelchair was brought out, but Johnson was eventually able to walk off the court.

''That's the first time I've ever retired in a match - I'm not going to leave in a wheelchair,'' he said.

Temperatures were in the high 80s, but Johnson said he's played in far worse conditions before with little problem.

For the 26-year-old Ito, it was just his third career Grand Slam win.

Johnson, 24, reached a career-high ranking of No. 49 this month after beating then-12th-ranked John Isner and 13th-ranked Ernests Gulbis in hard-court tuneups. The Californian made it to the third round of the 2012 U.S. Open.

''I'll be OK,'' he said. ''It's not the end of the world, but it feels like it right now.''