ISTANBUL, TURKEY- SEPTEMBER 17: Christiano Ronaldo of Real Madrid celebrates his goal against Galatasaray during UEFA Champions League Group B match at the Ali Sami Yen Area on September 17, 2013 in Istanbul, Turkey. (Photo by Burak Kara/Getty Images)Galatasaray AS v Real Madrid CF - UEFA Champions League
* Galatasaray overrun after strong start
* Madrid score five in the second half
* Bale makes substitute appearance (Adds quotes)
By Ece Toksabay
ISTANBUL, Sept 17 (Reuters) - Cristiano Ronaldo celebrated his contract extension by grabbing a second-half hat-trick as Real Madrid thrashed Galatasaray 6-1 in their Champions League Group B opener on Tuesday.
Real, who started with world-record signing Gareth Bale on the bench, went ahead in the 33rd minute against the run of play when midfielder Isco's shot went in off the post.
Forward Karim Benzema doubled their lead in the 54th, calmly slotting past Fernando Muslera, before Ronaldo netted after 63 minutes and added his second three minutes later with Bale, who had just come on, involved in the build-up.
Benzema scored again in the 81st minute, tapping the ball home following a cross from Ronaldo, before Umut Bulut got a consolation goal for the Turkish champions in the 84th.
But Ronaldo, who signed a three-year contract extension at the Bernabeu this week, completed his hat-trick in added time when he sailed past three defenders before firing a powerful left-footed shot into the net. It was his second Champions League hat-trick.
Real coach Carlo Ancelotti said his team had started with a lack of confidence.
"The result was great but it doesn't tell the story of the match," he said. "Galatasaray played very well in the first half. The match was easier after Isco's goal. Cristiano and Benzema gave a great performance.
"It's an important victory because when you start well we have an important advantage compared to other teams. It's important to win away the first game, of course it will be a boost in our confidence."
Galatasaray, who beat Real 3-2 in Istanbul in the Champions League quarter-finals last season but went out on aggregate, had started the match brimming with confidence and had the better chances in the first half-hour.
Brazilian midfielder Felipe Melo went close in the second minute with a powerful shot forcing a diving save from Ilker Casillas.
The Real captain was forced off in the 15th minute after sustaining an apparent rib injury in a collision with team mate Sergio Ramos. Medical staff taped up his ribs but it was soon clear he could not continue and he was replaced by Diego Lopez.
Ancelotti said Casillas's injury did not appear serious.
Lopez was called into action almost immediately, smothering the ball after a close-range effort from Melo and on the half-hour mark he pulled off a stunning save to keep out a header from Melo from a Wesley Sneijder corner.
Minutes later Real went ahead thanks to Isco, who was making his Champions League debut for Real. He has scored four goals and delivered three assists in nine starts in the competition, eight of them for Malaga.
"You're playing Real Madrid and you can't afford the slightest loss of concentration. If you make a momentary mistake they will punish you immediately," Galatasaray coach Fatih Terim said.
Real have now scored at least once in each of their last 26 Champions League games. Only Barcelona have had a longer run, of 29 from 2009 to 2012.
Galatasaray suffered a blow shortly before half-time when Didier Drogba, Chelsea's hero in the 2012 final, came off with a shoulder injury. He was replaced by Nordin Amrabat in the second half. Terim said he would have hospital tests on Wednesday.
The home side had a golden opportunity to drag themselves back into the match in the 47th minute when a cross from Amrabat found Burak Yilmaz, who headed narrowly wide from close range.
Benzema punished Galatasaray for the miss seven minutes later, racing in on goal after Angel Di Maria knocked the ball into his path and beating Muslera in a one-on-one.
Galatasaray's hopes of a comeback were shattered in the 63rd minute when Isco found Ronaldo, who slammed the ball into the roof of the net.
"It was a heavy defeat but ultimately it means just losing three points. What is important is to keep up morale and spread how we played in the first half-hour over the whole match," Terim said.
Bale, who cost 100 million euros ($133 million) when he joined from Tottenham Hotspur this month, came on as a substitute shortly afterwards and helped his 94-million-euro teammate score his second.
Ancelotti said he did not start with Bale because he was not 100 percent ready and wanted to prepare him for Sunday's La Liga game against Getafe.
"Bale, I didn't want to take a risk because he didnt have a normal pre-season," Ancelotti said.
(Reporting by Daren Butler; Editing by Ken Ferris and Stephen Wood)