DOHA (AP) ― Defending champion Roger Federer extended his winning streak to 20 straight matches Thursday to reach the semifinals of the Qatar Open, despite being stretched by Andreas Seppi of Italy to three sets in a 6-3, 5-7, 6-4 victory.

The third-ranked Federer set up a rematch of last year’s semifinal against Jo-Wilfried Tsonga. The sixth-ranked Frenchman defeated Albert Ramos of Spain 6-2, 6-1.

Rafael Nadal also reached the semifinals, playing some of his best tennis this week in dispatching of Mikhail Youzhny 6-4, 6-4. He will face Gael Monfils, who won the last four games in each set to beat Viktor Troicki of Serbia 6-2, 6-3 and ensure that the top four seeds reached the semifinals for the second year in a row.

The 30-year-old Federer, who is 26-3 at the Qatar Open and is looking to win his fourth title in Doha, hasn’t lost since falling to Novak Djokovic in the U.S. Open semifinals.

“I thought I started well. I was able to play aggressive, not giving too much rhythm. He was obviously missing more shots due to that,” Federer said. “I thought he was a better player in that second set and deserved that set, and then the third set was close. I had my chances, he had his chances. But I was able to take mine.”

Federer, who had never dropped a set against Seppi in seven wins, jumped out to a 3-0 lead in the first set and appeared on the way to an easy victory. But he fell apart in the second set, going down 5-1 after a string of unforced errors coupled with improved returns from the 38th-ranked Italian. Federer came back to even the set at 5-5 but Seppi took the last two games, leveling the match when the 16-time Grand Slam champion hit his return long.

Federer fully recovered in the third, breaking Seppi to go up 2-1. He saved two break points to take control at 5-3 and closed out the match with a forehand winner into the corner.

Federer chalked up his struggles in the second set to the return game of Seppi and his own defensive lapses.

“I definitely think Seppi, he’s a good return player,” Federer said. “When you give him too many second serves, he can take advantage of that … I had some chances to maybe not get broken a couple of times. So often I had the lead in some of the service games where I did get broken eventually, and I hope I can sort of not make that happen so often.”

Facing his toughest test yet in the former eighth-ranked Youzhny, the second-ranked Nadal went on the attack from the beginning.

He stretched the Russian with his forehand but failed to convert several break points early on. He finally broke Youzhny in the fifth game to go up 3-2 and held on to win the first set. Nadal broke Youzhny twice in the second set to go up 4-1, before Youzhny earned his only break of the match to make it 4-3.

Nadal then struggled to serve out the match, having to save four break points before clinching the victory with a volley into an empty court.

“I think I played my best match here today,” Nadal said. “Doing a lot of things very, very well. ... For moments, I enjoyed a lot playing tennis tonight.”

After meeting up with Schalke’s Spanish forward Raul Gonzalez, who watched the match with his teammates, Nadal told reporters he is also feeling happier about his game and appeared to have the spark back following a year in which he repeatedly lost to Novak Djokovic and bemoaned his play as sometimes too predicable.

“Well, I think I am fresh mentally. That’s important,” Nadal said. “But I want to do it well, and I really am motivated to try to play well ... I am in the right way. That’s what I can say. My feeling is I am in the really right way. We’ll see what’s going on. I am doing much better than what I felt before this tournament, so that’s a really positive thing.”

Tsonga, who struggled to three-set wins in his first two matches, broke Ramos in the first game and was never seriously challenged. He broke Ramos a second time to go up 5-2, and then jumped out to a 4-1 lead in the second set, repeatedly pumping his fists when he hit winners and dancing as he closed out the match.

“I was very aggressive at the very beginning with my play so I did well today,” said Tsonga, who joked this week that he was normally a “diesel engine” who starts slow.

Tsonga said he was looking forward to a rematch with Federer, whom he lost to last year in straight sets. Federer was 6-2 in 2011 against Tsonga, but lost to him in five sets at Wimbledon.

“It’s (always) a pleasure and an honor to play against this guy,” Tsonga said. “When you beat him, you know you can do it and that’s it. So of course when you are going on the court the next time, you are a bit more confident. But it’s all the time difficult anyway to play against this guy because when he plays his best level he’s better than every player.”