* Scotsman struggles with wind, heat
* Defending champion turns back Mayer
* He faces Istomin in fourth round (Adds quotes, details)
By Larry Fine
NEW YORK, Sept 1 (Reuters) - Defending champion Andy Murray sailed along after winning a first-set tiebreaker to beat Florian Mayer of Germany in steamy conditions and book a berth in the fourth round of the U.S. Open on Sunday.
Murray struggled with the wind, the heat and his unorthodox opponent but put it all together in the tiebreak, winning it 7-2 and carrying on to a 7-6(2) 6-2 6-2 victory in high humidity on Arthur Ashe Stadium court.
"Very tough conditions today," third seed Murray said after eliminating 47th-ranked Mayer, who changes pace and disguises his strokes to lull the opposition.
"He's a very tricky opponent, plays a lot of strange shots. Takes your timing away and tough to get your rhythm, so I'm really glad to come through in three sets," added Murray.
Steamy conditions led the Scotsman to shed his trademark baseball cap, apply ice packs on his legs and an iced towel around his back.
"I was struggling breathing for most of the match," said Murray.
"It cooled down a bit toward the end, but at the end of the first set and in the second set it was extremely hot."
Mayer fought Murray to a standstill during the opening set as both players held serve with relative ease.
"I played a really good first set," said the 29-year-old German, who had dropped both previous meetings with Murray but took him to a pair of tiebreakers in their most recent match on clay this year in Madrid.
Murray rose again in this decider, winning the first four points to seize command.
"On the big points, that's why he's so good," said Mayer. "I think from the baseline I was the better player in the first set but I didn't take my chances. Sometimes on my second serve I made easy mistakes."
The Briton won 57 percent of Mayer's second-serve deliveries.
"From start of the second set my level dropped a little bit and he started to play," the German said. "It was too easy for him."
Wimbledon champion Murray will next play 65th-ranked Denis Istomin of Uzbekistan, a 6-3 6-4 2-6 3-6 6-1 winner against 20th seed Andreas Seppi of Italy.
Looking ahead, Murray said he would like to get off to faster starts than how he began against Mayer.
"I think I need to maybe start matches a little bit quicker," he said. "I was a little bit slow out of the box. Once I got going I started to strike the ball a little bit cleaner and I was moving my feet, getting in the right position."
On balance, Murray felt good about making his first ever defense of a grand slam title.
"Expectations are maybe higher, but there's not as much pressure on me to win," explained the Scotsman.
"I feel a lot more comfortable coming into these events than I did at this time last year." (Reporting by Larry Fine, Editing by Gene Cherry)