John Isner fired 16 aces to advance to the quarterfinals of the BB&T Atlanta Open with a 4-6, 6-3, 6-4 victory over Ruben Bemelmans on Thursday night at Atlantic Station.

The match was delayed for 25 minutes between the second and third sets when a circuit breaker overheated in a bank of lights in the northeast corner of the stadium court.

Isner, the No. 1 seed and a former standout at Georgia, will play 19-year-old Jack Sock on Friday, not before 9 p.m.

Isner has made it to the finals of the past two tournaments in Atlanta, losing twice to Mardy Fish.

That won't happen this year after Fish retired from his match with an ankle injury.

Leading Gilles Muller 6-4, 2-2, Fish almost stumbled head-first into the net post while trying to return a drop shot. As he turned his head to avoid the post, he stretched his right leg, resulting in the injury to the inside of the right ankle.

"It was either going to be my knee, it was going to be my ankle or it was going to be my head," he said. "Thankfully, it was my ankle. That would have hurt pretty bad."

Fish, the No. 2 seed, said the injury wouldn't be diagnosed until an MRI was performed Thursday night or Friday.

"This is hard because obviously I like playing here in Atlanta," he said.

Fish was examined by trainers immediately after the injury and returned to play. He won the game to take a 3-2 lead.

He called for a trainer during the change-over and again returned to play.

Muller served once. Fish watched the ball go by and then took a couple of steps toward the net. He bent over and hit the top of racket into the court a few times before reaching out his hand to Muller to retire.

Not rain, but heat

James Blake went down with a few games left in his second-round match against Matthew Ebden with a back injury. Blake was able to get up and complete the match, losing 7-6 (6), 4-6, 4-6. Ebden will play Muller in the quarterfinals.

"I didn't notice anything with him until later in the match," Ebden said. "It was extremely hot and extremely humid out there, some of the hottest conditions you can play on on the Tour, easily."

The ATP World Tour doesn't keep on-court temperatures, which are usually warmer than the air temperature, which was at least 90 degrees by the end of the match.

Fish played in the next match and said he didn't think the heat was that bad, saying Ebden should have played in Atlanta in 2010, when on-court temperatures surpassed 100 degrees in the semifinals.

Sock wins again

In a battle of young players, Sock defeated 22-year-old Steve Johnson 7-6 (5), 7-6 (5) to reach the first quarterfinal on the ATP World Tour in his career.

Neither broke the other's serve, with the duo combining for 19 aces.

"Hopefully I can ride the wave a little bit and keep going," Sock said.

Japanese matchup

Kei Nishikori will meet Go Soeda on Friday in the first matchup between Japanese players in the quarterfinals of an ATP World Tour tournament in the Open era (since 1968). Their match will take place no earlier than 4 p.m.

Nishikori, the No. 3 seed, defeated Ricardas Berankis 6-4, 6-7 (7), 6-4. Soeda, the No. 8 seed, defeated Igor Kunitsyn, 6-1, 7-5. Both matches were played Wednesday.