* Del Potro wins Sydney title
* Isner reclaims Auckland crown
* Muguruza wins maiden WTA title
* Nishikori beats Berdych at Kooyong (adds quotes)
By Nick Mulvenney
SYDNEY, Jan 11 (Reuters) - Argentine Juan Martin Del Potro served notice of his strong form ahead of the Australian Open when he crushed defending champion Bernard Tomic 6-3 6-1 in the final of the Sydney International on Saturday.
The 2009 U.S. Open champion simply overpowered the talented but inconsistent Australian on Ken Rosewall Arena, bringing the one-sided contest to a close with a booming service winner after 53 minutes to claim his 18th career title.
Playing near faultless tennis, Del Potro served with venom and unleashed powerful forehands and backhands that forced a string of errors from the overwhelmed Tomic.
"Finals never are easy, but I was surprise by my level tonight. I think I played great," said Del Potro.
"My forehand worked perfectly, I made a lot of winners, many aces, played good slices. Every long rally we played I won all of them."
Seeded fifth at Melbourne Park, the 25-year-old looks to be approaching the sort of form that secured him his greatest triumph at Flushing Meadows before a wrist injury nearly ended his career.
"In Melbourne playing five-set matches, the body has to respond 100 percent all the time," he added.
"I think I did good preparation during the off-season, and I will see what happens next week."
Tomic, who faces world number one Rafa Nadal in the first round of the Australian Open next week, admitted Del Potro had been "too good" when the pair shook hands at the net.
American John Isner earlier claimed his second Auckland Open title with a 7-6(4) 7-6(7) victory over an inspired Lu Yen-Hsun in a tight final in New Zealand.
While Lu ultimately came up short in his first final, Spanish qualifier Garbine Muguruza hammered Klara Zakopalova 6-4 6-0 in hers to clinch a maiden WTA title at the Hobart International.
Japan's Kei Nishikori was also a winner on Saturday when the lucky loser finalist beat world number seven Tomas Berdych 6-4 7-5 to win the Kooyong Classic exhibition tournament.
In Auckland, Isner crashed down 23 aces but it would take more than his big serve to corral the Taiwanese, who put on a brilliant display of shot-making in his first ATP final.
Lu, number 62 in the world, stunned David Ferrer to end the Spaniard's three-year reign as champion in the semi-finals but Isner held his nerve when it mattered to edge two tiebreaks and avoid another upset.
"I played exceptionally well today and I needed every bit of it to come out on top," said the American, who is seeded 13th for Melbourne Park and, like Del Potro, will meet a qualifier in the first round.
In Hobart, Muguruza's dominance and aggression reduced the experienced Zakopalova to tears of frustration in the second set of the 70-minute match.
The 20-year-old was all over Zakopalova's serve from the start and, having fended off one match point, the Czech conceded the contest with her third double fault.
"I can't explain in words how happy I feel," Muguruza said. "Last year I was injured for six months and it's amazing to be here."
It was the second day in a row that a qualifier had won her first WTA title at an Australian Open warm-up after Tsvetana Pironkova claimed the Sydney International women's title. (Editing by Patrick Johnston)