Mardy Fish battled a thyroid condition about a month ago while in Melbourne for the Australian Open, where he exited the tournament in the second round. Afterward, Fish said the thyroid problem was sapping his energy after about 30 to 45 minutes on the court.

Fish didn’t need much more time than that to win his second-round match Thursday afternoon at the Delray Beach International Tennis Championships. The 29-year-old American, who struggled in a five-set first-round win before losing a four-set match in the second round at the Australian Open last month, took only 54 minutes to defeat Brazilian and 2004 ITC champion Ricardo Mello 6-2, 6-1 in a duel between two former ITC champions.

So far, Fish, ranked No. 16 in the world, has not had to tax his energy reserve very much. His first-round match on Tuesday barely lasted 30 minutes when his opponent, No. 92-ranked Bjorn Phau retired trailing 5-0 in the first set, citing flu-like symptoms similar to those that caused No. 1 seed Andy Roddick to withdraw from the tournament on Monday.

Fish, the No. 2 seed here this week, said his “immune system has been down because of this thyroid thing,” but so far he has avoided the flu bug that some players apparently caught last week in Memphis, and the fatigue and muscle soreness that he recently has experienced as a result of the thyroid problem has not reared its ugly head this week.

“I feel great,” said Fish, who advanced to the semifinals last week at Memphis and is the only one of the top four seeds remaining at the ITC. “I couldn’t be happier with the way it’s gone so far this week. I played four matches last week (in Memphis), and every day is a better day than the day before.”

Three weeks ago, Fish said he wasn’t sure he’d even be able to play the Delray tournament, and even next week’s Davis Cup in Chile was up in the air.

“I felt terrible 20 days ago,” said Fish, who was the 2009 Delray ITC champion. “I was thinking maybe targeting Indian Wells to come back. That’s how badly I felt and how little tennis I’d played and how little fitness I’d done. We came to the conclusion that trying to get healthy was first and foremost. I definitely didn’t think I was going to feel like I do now.”

Fish even hinted after Thursday’s match that he feels he might be able to win here this week and be a repeat champion. Should he capture the ITC title, he would become only the fourth repeat champion — joining Jason Stoltenberg (1996, ’97), Jan-Michael Gambill (2001, ’03) and Xavier Malisse (2005, ’07).

His quarterfinal opponent Friday will be Colombian Alejandro Falla, who toppled Adrian Mannarino of France 6-3, 6-2 Thursday afternoon in the other ITC singles match.

“He’s a good young player,” Fish said of Falla, who won his three qualifying matches to earn a spot in the ITC main draw.

Fish needed five sets to beat Falla the last time they met at the Davis Cup in Colombia, so Fish knows he likely will not have as easy, nor as quick of a match in the quarterfinals.

“He has a great two-hand backhand, very similar to (Mello),” Fish said of Falla. “He seems like he’s hungry.”

Fish lives in California, but he has South Florida roots, having attended Boca Prep along with Roddick his junior and senior years of high school, and his parents live in Vero Beach, less than two hours north of Delray Beach. His grandparents also have been here to watch him play.

“That’s why this week is so much fun,” Fish said of the chance to see family.

So far, winning has been icing on the cake.

The only other afternoon action on the main Stadium court was a doubles match, won by the team of Christopher Kas/Alexander Peya, who defeated Benjamin Becker/Michael Kohlmann 6-4, 3-6 (10-8).

The feature match for Thursday’s evening session will match Juan Martin Del Potro of Argentina against Teymuraz Gabashvili of Russia, scheduled to begin not before 7:15 p.m.

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