By Chris Starrs, special to EmiratesUSOpenSeries.com

ATLANTA – The two-time defending champion of the BB&T Atlanta Open won’t get the opportunity to be the three-time titlist.

Late Thursday afternoon, Mardy Fish –- seeded second in the tournament -- retired during the second set of his match against Luxembourg’s Gilles Muller after injuring his ankle when he ran into a post chasing down a Muller drop shot. Fish claimed the first set 6-4 and was leading in the second 3-2 when he retired.

"I was sort of stumbling as I was going forward," said Fish. "I didn’t have my complete balance, as I was coming so close to the net, and I was going to go head-first into the post. So I did everything I could to bring my head back, and that brought my right leg out, sliding and jarring it back, and (the post) got the inside of my right ankle. It was either going to be my knee, my ankle or my head. Thankfully, it was my ankle."

Fish added that he planned to have his ankle checked out either Thursday night or Friday morning and said he had no idea of the extent of the damage, although he walked under his own power.

The 30-year-old Fish, who defeated John Isner in the finals of the Atlanta Open in 2010 and 2011, declined an invitation to join the U.S. Olympic team after a heart procedure in late May. Fish, ranked 13th in the world, earned a silver medal in the 2004 Olympics in Beijing, falling to Chile’s Nicolas Massu in the championship final.

He admitted frustration in dealing with injuries and ailments this year.

"It’s hard because I like playing here in Atlanta," he said. "I wasn’t playing well at the beginning of the year, and I worked so hard to get my confidence back and started playing well again in Miami, and then I had my heart problem and worked hard again to get back to Wimbledon. I was looking forward to the summer and coming here. I’ve never been to a place where I’ve been able to try three times in a row to win the tournament, and something else happens."

The unseeded Muller, ranked 64th in the world, will face Matthew Edben in the quarterfinals. He’s reached the quarters in three tournaments this season, including a finals appearance in May in Rome.

Let there be light

There was no rain Thursday, but Isner’s second-round match against Belgium’s Ruben Bemelmans was delayed for about 20 minutes between the second and third sets when a bank of lights in Stadium Court failed. The former University of Georgia star advanced to today’s quarterfinals with a 4-6, 6-3, 6-4 victory.

Fresh off his championship victory on Sunday over Lleyton Hewitt at the Hall of Fame Championships in Newport, R.I., and playing on a hard court for the first time since March, Isner posted 16 aces and recorded two of the match’s three service breaks.

"In a lot of my matches, I only need one break of serve, and that was the case tonight," he said. "I broke his first service game in the second and third sets and held on from there. I would have liked to have won those sets a little easier, but with the nature of my game and how I can serve, I especially like serving when I’m ahead. I think I’m a pretty good frontrunner, but I hope to return a little bit better (Friday)."

Although the delay may have frustrated fans, it came at an opportune time for Isner.

"The umpire told me they had to have the situation resolved, so I knew sooner or later it would be resolved," he said. "Prior to the lights going out, I was sort of rushing because I had to change my clothes completely – I was gross. What takes the longest is redoing my socks and ankle braces – that takes forever. I was panicking and spazzing out there, but the lights went off, and it gave me more time. I would rather deal with that than deal with rain."

Partners one day, foes the next

Proving that tennis often makes for strange bedfellows, wild-card entries Jack Sock and Steve Johnson teamed up Tuesday evening for doubles (losing to Alex Bogomolov Jr. and Gilles Muller) and then squared off against each other Thursday afternoon in the second round of singles.

Although Sock won in two sets, it was a tight match, with the Nebraska native defeating the former Southern California standout in two tiebreakers, 7-6 (5), 7-6 (5).

"I think we both served well," said Sock, who recorded 11 aces (five in the first set and six in the second) and won 87 percent of his first-serve points. "There were no breaks and only five break points combined between us. We did a good job on saving break points, and we both served well. We both could have returned a little bit better, but when it was crunch time or it was close or a big point, maybe I put a better ball in play or played the point better. It was a very close match, and I was happy to get out of there in two sets."

"The difference today was just a couple of points," said Johnson who recorded eight aces and got in 61 percent of his first serves. "It was frustrating because in both breakers I had a mini-break and then missed a forehand both times and in the second breaker missed two forehands. It’s a learning experience. It’s frustrating to lose, but Jack played well when it counted."

Sock and Johnson faced each other last November in an indoor match in Virginia, and both said teaming one day and facing off two days later is part of the game.

"With guys your age, you’ve got to know you’ll probably be playing them a lot," said Sock, who upset the seventh-seeded Bogomolov in the first round. "Obviously, he’s starting, and I’ve been playing for less than a year. We’re both up and coming, and we know we’ll see each other (again). We had a great time playing doubles together, and we’ve said we’re going to try to play a lot together in the future."

"It happens more than you think," said Johnson who advanced to the second round after posting his first ATP Tour victory on Monday, defeating Olympian Donald Young. "It’s something we’re used to. We’re friends off the court, but on the court, it’s all business. It’s the nature of the sport. You get used to it, but it’s tough because we know each other’s games pretty well, and in the end, he played the last couple of points better."

Sock advances to his first quarterfinals and will meet top-seeded Isner Friday not before 9 p.m. ET.

Bad knee, bad news for Blake

The good fortune that James Blake enjoyed in the first round of the tournament eluded him Thursday, as he was eliminated in the second round by Australian Matthew Ebden, 6-7 (6), 6-4, 6-4.

The 32-year-old Blake, who has been dogged by knee and shoulder problems, was seeking his first quarterfinal berth since last October. He seemed to be favoring his right knee throughout the match and late in the final game of the third set required a trainer’s help to deal with his ailing left shoulder.

"I didn’t notice anything until late in the match," said Ebden of Blake’s difficulties. "It was extremely hot and humid out there – probably some of the hottest conditions you can play under on the tour. I think by the end he may have had a little cramp somewhere or his back started seizing up. I don’t know, but it was really hot out there."

Citing fatigue, Blake – who won his 350th career ATP Tour match on Monday -- declined to speak to the media after the match.

The 24-year-old Ebden, who last year won the tournament’s doubles title with Alex Bogomolov Jr., then needed but two quick points to send Blake packing. Blake upset seventh-seeded Ryan Harrison, 1-6, 6-3, 7-5, in the first round on Tuesday.

Ebden, ranked 82nd in the world, has now qualified for his first quarterfinal since last October and will meet Muller.

History made in Atlanta

Kei Nishikori (seeded third) and Go Soeda (seeded eighth) will meet after 4 p.m. ET Friday, the first time in Open Era history (since 1968) that two Japanese players will compete in the quarterfinals.

Nishikori defeated Ricardas Berankis, 6-4, 6-7 (7), 6-4, in the second round, while Soeda bested Igor Kunitsyn, 6-1, 7-5, to reach the quarters.