World number one Roger Federer sealed his berth to the semifinals of the year-ending ATP Masters Cup in Shanghai on Tuesday with a last round-robin match to play, as the Swiss claimed a second straight win in group stage over in-form Croatian Ivan Ljubicic.

But Guillermo Coria's hope of qualifying for the semis was dented later the day after the fourth seed, who had been defeated by Ljubicic in Sunday's tournament opener, lost again to lie bottom of the four-man group.

Coria was beaten 7-5, 6-4 in a lacklustre showdown with his compatriot David Nalbandian on Tuesday as both of the players failed to tightly hold their own serves. With 14 double faults, the 23-year-old Coria surrendered in 1 hour and 48 minutes.

In the men's doubles play, reigning world number one Bob and Mike Bryan recovered from their sluggish tournament opener to see off former top-ranked pair Mark Knowles and Daniel Nestor 6-4, 6-4.

Earlier in the day, fifth-seeded Leander Paes of India and Nedad Zimonjic from Serbia and Montenegro claimed their second winin Shanghai, defeating seventh-seeded Australian duo Wayne Arthursand Paul Hanley 7-6(3), 7-6(6) in two hours.

In the men's singles, top-ranked Federer had to work hard as the two-time defending champion of the season-ending Masters Cup was forced into the tiebreaker in the decider before edging out at just the second match in Shanghai.

It took two hours and 17 minutes of the 24-year-old Swiss to defeat the in-form Croatian Ivan Ljubicic 6-3, 2-6, 7-6(4), with a forehanded shot out of Ljubicic's reach to take the tiebreaker.

After Federer's hard-earned victory on Sunday over Argentina's David Nalbandian, the last man having qualified for the elite eight-man field, nothing wrong from the tennis super star could bemore shocking, even another rare fightback and a tiebreaker.

With all the embarrassing backhand errors, weak returns and slow paces, Federer failed to overcome his sluggish form shown in Nalbandian's match, being broken twice in the second set after taking the first using an advantage of serving first.

Forced into the decider, Federer, who has compiled another outstanding season with an ATP-best 11 titles including two Grand Slam and a record four ATP Masters Series crown, should have shortened his work with Ljubicic in the ninth game before the Croatian saved two match points with some placement shots.

Ljubicic, who enjoyed his best season by winning two ATP titlesin eight finals and broke into the top ten for first time, then continually moved Federer to play close to the net and used errors from the Swiss to his own benefit before holding on to force a tiebreaker.

With the victory, Federer saved his near-perfect record at the Tennis Masters Cup, increased his winning streak to 33, and extended his season-best match record to 79-3.

The Shanghai event was the first in six weeks for Federer, who has already clinched the world No. 1 year-end ranking for the second year in succession. He last competed in Bangkok, winning his 11th title of the season on October 2.

Ljubicic, the 26-year-old sensation of the late-season, had earned a place in Shanghai thanks to his autumn efforts, including the indoor title win over the past month in Metz and Vienna as well as the final encounters in both the Masters Series Madrid and Paris two week ago.

The Croatian struck 10 aces and managed 36 winner, but committed 5 double faults and 41 unforced errors in his defeat to Federer.

"It was a very close match. I feel really upset because I felt I deserved this one," said Ljubicic at a post-match news conference.

"Today I felt like for the first time I was the better player on the court (against Federer), and that's why it feels frustrating because some shots just went out that they were not supposed to."

Before taking over control of the match, Ljubicic seemed to have problems moving during the first set.

"I had problems with him because he was playing just unbelievable," he said. "I think he played really high-class tennis in the first set, but I was also expecting his pace is going to drop because he dropped against Nalbandian, and I know he's not in top form physically.

"So I was just waiting my chances, and they came, and I took them."

"But I mean, Federer is not struggling. I just think that he's not top fit. He's not sharp as he usually is...I just tried to play my best tennis. It was almost enough. Unfortunately, it wasn't," added the Croatian.

The usually flawless Federer declined to agree with the negative comments on his performance, said: "I felt like this is exactly the match I expected."

"I expected a close match. I thought maybe he was even slightly the favorite because of what I've been going through the last few weeks. Obviously, I'm very pleased with the result because a win is a win no matter if it's two or three sets."

Federer called physiotherapist coming on court to treat his sore ankle at the beginning of the third set, which raised worries that he might retire.

"No...it (the Masters Cup) means very much to me (that I can play). It's also should show a message how much and how important this tournament means to tennis. For me, this is equaling like a Grand Slam," said the Swiss.