( CNN ) -- World No . 1 Roger Federer overcame an early scare to reach the semifinals of the Estoril Open in Portugal on Friday , but second-ranked Novak Djokovic 's defense of his Serbia Open title was ended by health problems . Federer was broken in the first game by French veteran Arnaud Clement but bounced back to win 7-6 ( 9-7 ) 6-2 as he continues his build-up to the French Open , which he won last year . Djokovic 's own hopes on the red clay of Roland Garros seem less positive after he was forced to retire having lost the first set of his quarterfinal 6-4 against young compatriot Filip Krajinovic at his home event in Belgrade . The top seed has been struggling all week with allergy problems and fatigue , according to his personal Web site . Federer is on track to win back the title he claimed on his only other appearance at Estoril in 2008 , but admitted he was not yet at his best ahead of Saturday 's semifinal clash with defending champion Albert Montanes . The Swiss star , who is closing on Pete Sampras 's record of 286 weeks at No . 1 , needed to save a set-point at 6-5 in the tie-break . `` I did n't play beautiful tennis . I had to play safe , which is n't something I 'm used to doing . I have always struggled against him and today you could see that , '' Federer told the ATP Tour Web site . Fourth-seeded Spaniard Montanes moved into the last four by beating Uruguayan eight seed Pablo Cuevas 6-4 6-3 . He has lost all three previous encounters with Federer , two of them on clay . In the other semifinal , fifth seed Guillermo Garcia-Lopez takes on world No . 133 Frederico Gil , having progressed when fellow Spaniard Alberto Martini retired due to illness . Gil went through after beating compatriot Rui Machado 4-6 7-6 ( 7-1 ) 6-3 in the first all-Portuguese quarterfinal at an ATP World Tour tournament . The 18-year-old Krajinovic , a wildcard entry , will play American third seed Sam Querrey for a place in the Belgrade final . Querrey defeated Russian Igor Andreev 6-3 2-6 6-4 to reach his fourth semifinal this season . The winner of that match will play either big-serving American second seed John Isner or Swiss No . 4 Stanislas Wawrinka . In Munich , top seed Marin Cilic reached the semifinals of the BMW Open by beating sixth-seeded Spaniard Nicolas Almagro 6-4 6-7 ( 2-7 ) 6-2 . The Croatian world No . 11 will next play fifth seed Marcos Baghdatis of Cyprus , who beat 2007 champion Philipp Kohlschreiber 6-3 6-4 . Second seed Mikhail Youzhny , the runner-up for the past two years , also moved into the semis with a 6-1 6-3 win over Czech Jan Hajek . The Russian will next face world No . 45 Philipp Petzschner , who upset defending champion and third seed Tomas Berdych 1-6 6-3 6-4 .
World No . 1 Roger Federer reaches semifinals of Estoril Open in Portugal . Swiss star , the 2008 champion , overcomes slow start to beat Arnaud Clement . Novak Djokovic 's Serbia Open title defense ends when he retires in quarterfinal . World No . 2 has been suffering with allergy and fatigue at his home event in Belgrade .