- Roger Federer withdraws from Qatar Open with back injury
- Three-time defending champion was due to play Jo-Wilfried Tsonga in the last four
- Top seed Rafael Nadal lost in straight sets to France's Gael Monfils in second semifinal
- Andy Murray sweeps into semifinals of Brisbane International
Roger Federer pulled out of his defense of the Qatar Open Friday with a back injury before his arch-rival Rafael Nadal was beaten in the semifinals of the $1 million tournament by Gael Monfils.
Federer, the three-time defending champion at the season-opening tournament, was due to face France's Jo-Wilfried Tsonga for a place in the final.
Monfils, who brushed aside top seed Nadal in straight sets, 6-3 6-4, will now play his compatriot for the first title of the 2012 ATP Tour season.
Swiss maestro Federer had nursed back spasms during his wins over Greg Zemlja and Andreas Seppi and undergone treatment between matches as well as taking painkillers.
But with the Australian Open looming on Monday week, the 16-time grand slam champion decided against risking the injury against Tsonga.
"I hurt my back in my second round match (against Zemlja) and I don't see any improvement today," he told gathered reporters.
"I've played through two matches in pain and don't think it's the right thing to risk anything more now. It's the only right decision to make.
"It's only the second time I have pulled out from a tournament, and I have never pulled out during a match. It's a sad moment for me, the tournament, and the fans, but health comes first."
Federer sounded a cautious note ahead of the first grand slam of the season.
"Although it's not very good it's not crazy bad," he said.
"I have had bad backs on the past, but this is not very good, otherwise I would definitely be playing," added Federer, who is on a 20-match unbeaten run on the ATP Tour.
In the later action on court, a single break of service saw fourth seed Monfils take the opening set.
Nadal hit back to lead 4-1 in the second set before Monfils conjured up five games in a row to compete a stunning victory in one hour 36 minutes.
Tsonga holds a 2-1 career winning record over Monfils, but he lost their last encounter in Montpellier in 2010.
Earlier, Andy Murray raced into the semifinals of the Brisbane International with a straight sets win over Marcos Baghdatis of Cyprus.
World No. 4 Murray beat Baghdatis 6-2 6-2 and will play 19-year-old Australian hopeful Bernard Tomic in the last four in Queensland.
Tomic beat Uzbekistan's Denis Istomin 6-3 7-6 in their quarterfinal match.