BRISBANE, Australia — Top-seeded Novak Djokovic was upset in the first round of his season-opening tournament Tuesday, losing in straight sets to Latvia's Ernests Gublis at the Brisbane International.

Djokovic, who started the week with a chance of replacing Roger Federer at No. 2 in the world rankings, lost 6-4, 6-4 to the 20-year-old Latvian who finished last year at No. 53.

"I'm not panicking. I'm not seriously a big doubt. The Australian Open is my highest goal for this part of the year," Djokovic said. "It's not the end of the world. It's the first match, so I'm not going to be pessimistic."

Djokovic dropped only one set as he won his first Grand Slam title at last year's Australian Open, beating Federer in the semifinals and Frenchman Jo-Wilfried Tsonga in the final.

The 21-year-old Serb had been a hot favorite to improve to 3-0 against Gulbis, both previous wins coming in quarterfinals last season at the French Open and at Cincinnati.

But he said he had trouble finding rhythm on his serve with a new racket and that contributed to him being broken in the fifth game.

Both players dropped serve twice to open the second set before Djokovic held for a 3-2 lead, raising both arms in mock triumph when he earned a game point with an ace.

But he was in trouble again in the ninth game, facing two break points at 15-40 and dropping serve when he lost a scrambling rally.

Gulbis closed out at love, sealing the win in 1 hour, 28 minutes with an ace, his fourth of the match.

He said the upset was simply a matter of him handling the added pressure of the season's first match better than his rival, rating his own performance down along with Djokovic's.

"It's one of my best wins — (but) best games, I don't think so," Gulbis said. "I had some beautiful losses, so yeah, it's the best win but not my best game."

Playing for the first time with a Head racket after switching from Wilson, Djokovic's first serve percentage dropped to 48 and he was broken four times. He said he needed time to get used to the changes.

"I didn't play well. I served really, really bad and couldn't get any rhythm," Djokovic said. Gulbis "is young, he's talented and he's got nothing to lose and he used his opportunities."

Djokovic said the setback gave him more time to prepare for the Australian Open, starting Jan. 19 in Melbourne.

"Let's look on the bright side, it's only the start of the year and the biggest tournament is coming up."

The loss did have ramifications for Djokovic, who could have moved to a career-high No. 2 ranking if he'd been successful in Brisbane and Federer flopped at Doha.

He said the rankings calculations didn't cause him any extra nerves.

"It was not playing a big role at all," Djokovic said. "It was just a matter of me playing well or not."

Second-seeded Tsonga was playing Argentina's Agustin Calleri later Tuesday.

In other matches, fifth-seeded Tomas Berdych of Czech Republic beat Australian Brydan Klein 6-0, 6-4 and Florent Serra of France ousted former No. 1 Juan Carlos Ferrero of Spain 6-3, 7-6 (3).

American qualifiers Amer Delic and Bobby Reynolds both went out. Croatia's Mario Ancic overcame Delic 7-6 (2), 6-7 (4), 7-6 (6) and Japan's Kei Nishikori beat Reynolds 6-3, 6-2.

On the women's side, second-seeded Victoria Azarenka missed two match point chances before sealing a 7-6 (4), 7-5 win over Jarmila Gajdosova of Slovakia and No. 3 Marion Bartoli of France beat Hungary's Melinda Czink 5-7, 6-3, 6-1.

Azarenka, runner-up at the Gold Coast tournament last year which was a forerunner to the joint ATP-WTA Brisbane International, thought she'd won on her first match point but was on the receiving end of an erroneous out call on her deep forehand.

She broke Gajdosova at love in the next game to avoid another tiebreaker and reach a quarterfinal match against either Australia's Samantha Stosur or Lucie Safarova of Czech Republic.

Tathiana Garbin of Italy rallied for a 4-6, 6-4, 6-2 win over Russia's Alisa Kleybanova.