By Steve Gorten

KEY BISCAYNE – It doesn’t happen for John Isner as often as it does Roger Federer or Rafael Nadal.

In fact, Isner said, he can probably count on one hand the number of times he’s gotten into a groove like he did Thursday at the 2015 Miami Open.

How great of a groove was he in during his 6-4, 6-3 quarterfinal win against Kei Nishikori?

“[His] serve, I didn’t have a chance,” said Nishikori, the world’s fifth-ranked player. “His forehand, also his backhand — he hit some winners from back of the baseline. I think he [could] close his eyes, he was hitting so many winners. I couldn’t really stop him.”

Through eight games Thursday, they were equal. Isner was even down love-30 serving at 4-4.

Then the 6-foot-10 American claimed 11 consecutive points and 19 of 21, collecting confidence with every ace and winner he swatted on stadium court. Across the net, the highest-ranked Asian in ATP Tour history, and a semifinalist here last year, looked defeated long before he actually was in a mere 70 minutes.

At set-point in the first, Isner ripped a forehand that hit the top of the net and gently dropped on the other side.

“That took a lot of pressure off me,” Isner said. “I just got really, really comfortable and confident from that point on. I played the break points well and I took care of my serve extremely well.”

Isner added 13 aces Thursday to his previous tournament-leading total of 47 in three matches. He won 78 percent of points on his first serve, 81 percent on his second serve. He didn’t face a single break point, and has won all five break-points against him in 48 service games here.

Nishikori, who lost just 10 games combined in the tournament before Thursday, and broken serve 15 times in 23 games, said it’s the most he’s ever been handcuffed by an opponent’s serve.

“He has more height, so it felt like it’s coming from somewhere different,” Nishikori said, smiling. “When you play with big servers, it’s not fun,” he noted. “It’s never easy to play against those big servers.”

Isner, the highest ranked American in the world at No. 24, became the first American to reach the semifinals of the Miami Open since Mardy Fish did in 2011. Andy Roddick is the last U.S. player to win the tournament (2010).

“I was in this situation last week in Indian Wells, but it didn’t last too long. I was the last American [left in the tournament] for a few hours,” said Isner, 6-0 in his career in ATP Masters 1000 quarterfinals. “It just so happens I’m the final American right now. Yeah, I like being in this position, but I’m not really thinking about that. I’m worrying about trying to really win this tournament. I believe I can do it, but it’s so tough. There are [four] incredible players left, and I’m one of them. I’m proud of that.”

The tournament’s No. 22 seed, Isner advanced to the quarterfinals by beating fifth-seeded/world No. 6 Milos Raonic 6-7 (3), 7-6 (6), 7-6 (5) Tuesday night, in a match that lasted two hours and 43 minutes, spilling over into Wednesday.

“You know, a lot of times, just to get to a situation, you have to win a match that maybe I couldn’t, shouldn’t have won, which was my last match,” he said.

Isner then teamed with Sam Querrey late Wednesday afternoon to beat Jean-Julien Rojer of Netherlands and Horia Tecau of Romania 6-3, 6-4 in doubles. That match “turned out to be perfect practice in my preparation for [Thursday],” Isner said. “I felt great out there physically. I was a little tired [Wednesday], but today I felt great.”

“I had a lot of things going for me,” he added. “I pulled out a match in the round of 16, one that I easily could have lost. So that gave me some confidence. Then [Thursday], I played the best match I’ve played all year – hands down.”

Isner beat his second consecutive top-10 opponent for the first time since Cincinnati in 2013. Before Tuesday, he had lost 11 in a row against top-10 opponents.

Less than a month ago, he arrived at Indian Wells on a four-match losing streak – a stretch that included two Davis Cup singles rubbers. Isner said he feels that second Davis Cup loss, to Andy Murray, was a positive because “I definitely played the right way.”

“As disappointing as that whole week was for me, I felt like I may have turned my season around a little bit. In fact, Jim Courier told me that,” Isner said.

“I got to California and really had a nice long car ride with Justin from [the airport in Los Angeles] to Indian Wells,” he added of his coach Justin Gimelstob. “We spoke about a few things and he got me in a very, very good mindset. It’s not that I’ve been working harder. It has nothing to do with that. It’s all mindset. I’ve always believed in everything Justin has told me up to this point, and I think things are starting to pay off a little bit.”Djokovic said after his fourth-round win that he was worn down. After his win Thursday, though, “I felt physically pretty good. Mentally, also tough,” he said. “I regained my strength as I had a day off, which I knew was going to serve me well. Now, playing again [Friday], it’s going to be a huge mental challenge for me. But I look forward to it. This is the final stages of a big event, and this is what I came here for, to play the big matches.”