World number one Rafael Nadal, who has been struggling with a heavy cold, worked hard for a 7-6(3), 0-6, 6-3 win over Slovakia's Lukas Lacko to reach the quarterfinals of the Qatar Open on Wednesday.

Nadal's second-set slump was only the 10th time in 575 career matches that Nadal had lost a set at love and the first time since February 2009.

The Spaniard won just 11 points in the second set, before he won the first three games of the deciding set.

Nadal hit six aces overall for victory in two hours and 19 minutes and will next face Latvian fifth seed Ernests Gulbis who enjoyed a 6-3, 6-1 win over Croatia qualifier Antonio Veic.

Second seed Roger Federer also had to fight unusually hard for the second day in a succession before reaching the quarterfinals with a 7-6(5), 7-5 win over compatriot and close friend Marco Chiudinelli.

The 16-time Grand Slam champion needed a tiebreaker in the first set and a strong baseline game in the second to register his second straight win in Doha, where he is looking for his third tournament title.

"It was really difficult today, under the circumstances," admitted Federer. "Playing such a good friend is so rare for me. I'm happy the match is in the past and I can look forward."

Earlier Jo-Wilfried Tsonga continued his comeback from a knee injury by reaching the quarterfinals as well.

The former world number six from France did that with an unusual finish in which two successful Hawkeye decisions in a row helped him to a 6-2, 6-4 win over Sergei Bubka, the son of Olympic gold medal pole vaulter Sergey Bubka.