Nadal is the reigning French Open champion, was last year's Wimbledon and U.S. Open runner-up, and was a Doha finalist two years ago.
The three-time overall and reigning Doha champion Federer was tested by eighth-seeded Italian Andreas Seppi on Day 4, as the iconic Swiss posted a hard-fought 6-3, 5-7, 6-4 decision on the hardcourts at Khalifa International Tennis Complex.
"It was tough, I started well and was able to take control. In the second set he started to hit the ball better and in the third it was close," Federer said.
Federer, who dialed up 12 aces and broke Seppi's serve five times en route to victory, is now 8-0 lifetime against the Italian, who had never taken a set off the Swiss before Thursday. Federer also bested Seppi here in Doha three years ago.
The Swiss superstar has now won his last 20 matches overall, dating back to the latter stages of last season when Federer rattled off 17 consecutive victories.
The 16-time major champion Federer beat Russian Nikolay Davydenko in last year's Doha finale.
The second-seeded former world No. 1 Federer will battle third-seeded Frenchman Jo-Wilfried Tsonga here on Friday. The former Aussie Open runner-up Tsonga leveled helpless Spaniard Albert Ramos 6-2, 6-1 in 67 minutes on Thursday.
Federer beat Tsonga in the final at last year's season-ending ATP World Tour Finals in London and also bested the Frenchman in the semis here in Doha in 2011.
"I'm looking forward to playing him," Federer said of Tsonga. "I hope we can live up to expectations."
The aforementioned Monfils capped Day 4 with an easy 6-2, 6-3 victory over fifth-seeded Serb Viktor Troicki.
Monfils, who was a Doha finalist back in 2006, will meet Nadal for a 10th time, with the mighty Spaniard comfortably leading the lifetime series, 8-1. The acrobatic Frenchman, however, was a quarterfinal victor here in Doha two years ago.
Clijsters rolls; Petkovic exits Brisbane
BRISBANE, Australia - Former world No. 1 and reigning Australian Open champion Kim Clijsters reached the semifinals, while second- seeded Andrea Petkovic went by way of a quarterfinal upset Thursday at the season-opening Brisbane International tennis event.
The fifth-seeded Clijsters cruised past Czech left-hander Iveta Benesova 6-3, 6-2 on the hardcourts at Queensland Tennis Centre.
The Belgian star Clijsters captured the title here two years ago.
"I'm very satisfied with the way that I played," Clijsters said. "In my previous match I lost my focus a bit. I was really focused on not letting that happen today."
The four-time Grand Slam champion Clijsters will face Slovakian veteran Daniela Hantuchova in Friday's semifinals. Hantuchova received a walkover into the final four when Serena Williams pulled out of the draw on Wednesday due to an ankle injury.
Clijsters is a laughable 9-0 lifetime versus Hantuchova and hasn't dropped a set in their last seven meetings.
The other semi will pit third-seeded French Open runner-up Francesca Schiavone of Italy against Estonian Kaia Kanepi. The 2010 Roland Garros titlist Schiavone fought off two match points and came back to beat sixth-seeded former No. 1 Jelena Jankovic of Serbia 5-7, 7-6 (7-2), 6-3, while Kanepi upended Petkovic 6-1, 7-6 (9-7) on Day 5 here.
Schiavone is 4-2 all-time versus Kanepi, but the Estonian captured their last meeting in Moscow last season.
A disappointed Petkovic was last year's Brisbane runner-up to Czech slugger Petra Kvitova.
"I just played really bad in the first set," Petkovic said on Thursday. "My footwork was very sloppy, and afterwards I started moving a little better, but when somebody is hitting as hard as Kaia -- and she was serving incredible -- then you really have to be on point with your footwork."
"She played really, really well and I have to say, if she keeps playing like this she'll be a force."
Simon, Dolgopolov onto semis
BRISBANE, Australia - Seeds Gilles Simon of France and Alexandr Dolgopolov of Ukraine were a pair of quarterfinal winners Thursday at the 2012 season-opening Brisbane International tennis event.
The second-seeded world No. 12 Simon got past Colombian Santiago Giraldo 7-6 (7-2), 6-4, while the No. 3 seed Dolgopolov came from behind to beat sixth- seeded Czech veteran Radek Stepanek 4-6, 6-3, 6-3 on the hardcourts at Queensland Tennis Centre. Stepanek captured the title here three years ago.
Simon and Dolgopolov, who's currently ranked a career-high No. 15 in the world, will square off in Saturday's semifinals.
The quarterfinals will wrap up here on Friday when top-seeded Andy Murray faces fellow Australian Open runner-up Marcos Baghdatis and young Aussie favourite Bernard Tomic meets Uzbekistan's Denis Istomin. Murray was last year's Aussie Open bridesmaid, while Baghdatis reached the final in Melbourne back in 2006.
Lisicki bows out in Auckland
AUCKLAND, New Zealand - Top-seeded Sabine Lisicki was forced to retire from her quarterfinal match on Thursday at the season-opening ASB Classic.
Lisicki, who survived a three-set battle on Wednesday to advance, dropped the first set to fellow German Angelique Kerber and was trailing in the second set when she bowed out with an abdominal injury.
"I have something with my abdominals; it hurts when I serve," Lisicki said after the match. "I kept fighting. I tried it. Unfortunately, couldn't finish. I knew if I pushed it too much it would take two weeks to recover."
Lisicki, a two-time winner last year and a semifinalist at Wimbledon, is slated to play next week in Sydney before the Australian Open starts on January 16.
Kerber, who was ahead 6-4, 4-3, will next face fourth-seeded Italian Flavia Pennetta, who advanced with a 6-2, 6-1 rout of Russia's Elena Vesnina. Pennetta was a finalist at this event two years ago, while Vesnina was the runner-up back in 2009.
Third-seeded Svetlana Kuznetsova of Russia handled Italian Sara Errani, 6-4, 6-3, and will meet China's Zheng Jie in the semifinals. Zheng moved on with a 6-2, 6-3 win over Lucie Hradecka of the Czech Republic.
Tipsarevic, Almagro reach quarters
CHENNAI, India - Top seeds Janko Tipsarevic and Nicolas Almagro secured berths in the quarterfinals at the season-opening Chennai Open tennis event.
The No. 1 seed and world No. 9 Tipsarevic trounced Indian wild card Yuki Bhambri 6-1, 6-3, while the 10th-ranked Almagro advanced via walkover when his scheduled second-round opponent, Belgian Steve Darcis, pulled out of the draw on the hardcourts at SDAT Tennis Stadium.
The Serbian Tipsarevic will face Belgian wild card David Goffin in Friday's quarters, while the Spanish Almagro will take on Japanese qualifier Yuichi Sugita.
Almagro has reached the quarterfinals this week without having played a match. He, like Tipsarevic, received a bye in the opening round prior to his walkover on Day 4.
In other second-round action here, Goffin grounded German Andreas Beck 4-6, 6-4, 6-2 and Sugita stopped Taipei's Yen-Hsun Lu 7-6 (7-5), 6-4.
Friday's other quarterfinals will pit third-seeded and defending champion Stanislas Wawrinka against Japanese qualifier Go Soeda and fourth-seeded rising Canadian Milos Raonic versus Israeli Dudi Sela. The Swiss slugger Wawrinka topped 2007 champion Xavier Malisse in last year's finale here and was the Chennai runner-up in 2010.