John Isner’s toughest opponent in his 6-3, 6-7(7), 6-4, 6-3 second round win over Jarkko Nieminen of Finland was the let sensor -- and his own head.

The 6-foot-9 North Carolina native became uncharacteristically argumentative with the chair umpire, who allowed what he thought were faulty let calls stand.

“I got a little frustrated with her,” Isner said following the match. “But it wasn't at her, it was more directed at the device, which I think sometimes malfunctions.”

He spent much of the second and third set looking alternately angry and dejected, and on the verge of a mental implosion. But it is a credit to Isner’s progression and maturity as a player that he was able to weather the storm inside his head and get a critical break to win the third set. He then used that momentum to play lights out tennis for a fouth-set win and a spot in the third round.

Isner started off the match playing with confidence, taking a 3-0 lead and hitting winners from all over the court. Nieminem struggled to even get a point off Isner’s serve throughout the first set.

The second set played out as is typical of an Isner-Nieminen match, with each player holding serve and sending the set into a tie-break. In the four times both men have played each other, at least one set has ended in a tie-break situation.

It was in the tie-break that Isner’s game and mental state began to unravel. At 5-4, with Isner serving, the chair umpire made a late let call after Isner and Nieminen exchanged returns and Isner had hit a clean winner. After arguing lengthily with the umpire, he was forced to replay the point.

“I think it happens when it's windy sometimes,” Isner said, referring to what he felt was an incorrect call by the let sensor. “I don't know exactly what it is, but, yeah, it happened certainly on a big point in that second set, too.”

Frustrated, he replayed the point angrily and, after hitting one pounding baseline shot after another, lost control and sent a return way long.

The entire incident seemed to unnerve him and he allowed Nieminen to take the following three points and win the second set. Isner, who wears his emotions on his sleeve - often to his detriment - screamed and pulled on his shirt.

“I mean, it's something I need to work on,” Isner said of his negative body language. “I need to try not to get too high and try not to get too low. Certainly in today's match there were some low points and it affected me a little bit.”

At 1-2 in the third set, with Nieminen serving, the umpire again made a late let call. A rattled Isner dropped the next two points. But he regrouped to bring Nieminen to deuce and the players exchanged Ad points before Nieminen eventually held for the game.

Isner allowed himself to fall into a 15-40 hole on his serve in the following game, but instead of crumbling, he found some fire and battled back to hold.

“Yeah, the third set was a little tricky,” Isner said. “I was down two or three breakpoints I know in that third set. Something clicked at 5‑4 me, 40‑15 him serving. I reeled off four straight points, I hit four great forehands and won that set and had the momentum from there.”

He used the momentum to play lights-out tennis in the fourth set and closed out the match, overcoming both tough calls and his own demons to reach the third round of the US Open for the fourth year in a row.

Though Isner often seems disheartened and tough on himself in his matches, shaking his head and slumping his shoulders when he makes mistakes, he always seems to be able to fight the negativity and tough his way through it.

“Yeah, I probably could have exhibited better body language, then again, I felt like I was in control,” he said. “But the thing is, if I am being a little negative, if my body language is a little negative, I always have my serve to rely on to at least keep me in the match. That's what happened in that third set, I was a little ticked off not winning the second set, but I was holding serve. Things eventually started rolling my way.”

Isner served up 23 aces in the match. He will face the winner of the Philipp Kohlschreiber/Benoit Paire match in the third round.

“If I play Kohlschreiber, I've played him a few times. I've played Paire once. I've won those matches,” Isner said as he sneaked glances at the live scoreboard displaying the match of his two potential opponents. “I'll take the court very confident. No matter who I'm playing, I'm going to take the court very confident."

• John Isner Bio
• Jarkko Nieminen Bio