Defending US Open champion Andy Murray was made to sweat by Argentina's Leonardo Mayer on his way to a 7-5 6-1 3-6 6-1 victory.
Murray fired 28 winners and made just 22 unforced errors to Mayer's 49 while scampering down shot after shot against his South American rival in a match-up of players who were born on the same day.
The world No 3 dropped a set for the first time in the tournament, but still came through in two hours and 41 minutes on a steaming hot day at Louis Armstrong Stadium.
There were signs Murray was in for tough workout when he needed almost an hour to clinch the first set, but he seemed to back in control when he raced through the second.
At 81 in the rankings the Argentinian was not expected to pose too much danger to the third seed.
But a sluggish Murray had his work cut out to hold off Mayer, and it was certainly his opponent who was doing most of the attacking.
Mayer broke Murray's serve and calmly held all of his own service games to push the match into a fourth set.
However, Murray eventually pulled away as he moved up another level to secure victory.
"It was a very tough match," Murray said. "Both of us ran a lot and it was very humid. I'm just glad I was able to put it together and pull it out in the fourth set.
"I was serving a little better as the match went on. He's a big hitter, I had to defend a lot. I think he played some really good tennis. It made for an entertaining match."
The reigning Wimbledon and Olympic champion will next face Florian Mayer for a place in the last 16.
The German defeated US qualifier Donald Young in straight sets, 7-5 6-3 6-4.
By (Sky Sports)