KEY BISCAYNE, Florida, 29 March 2004 — World No. 1 Roger Federer, fighting off illness and determined Russian rival Nikolay Davydenko, struggled into the third round of the WTA and ATP Masters Series event here Saturday.

Federer won his 12th match in a row and improved to 23-1 on the season by rallying from a break down in the third set for a 6-2, 3-6, 7-5 victory. He will face Spain’s 34th-ranked Rafael Nadal on Sunday for a round of 16 berth.

“Very difficult. I didn’t think I was going to turn this around,” Federer said. Three-time defending champion Andre Agassi opened with a 6-0, 7-6 (7-1) victory over Argentina’s Mariano Zabaleta at the $6.5 million hardcourt event, where every seed received a bye into the second round.

Women’s second seed Venus Williams also struggled before beating China’s Zheng Jie 6-4, 6-4 while Australian Open runner-up Marat Safin of Russia was ousted by American Vince Spadea 7-6 (9-7), 6-7 (4-7), 6-4.

Brazilian three-time French Open winner Gustavo Kuerten lost to 156th-ranked Argentine qualifier Juan Monaco 6-1, 3-6, 7-5 and 160th-rated Czech qualifier Jan Hernych beat Australian 11th seed Mark Philippoussis 6-4, 6-2.

Federer, 22, has won three ATP titles this year, including Dubai and Indian Wells this month and his second Grand Slam crown at the Australian Open, while 54th-ranked Davydenko, 22, had won only three matches all year.

Federer, who missed three days of practice with fever and vomiting, hit seven aces and 27 winners but his 36 unforced errors were 22 more than the Russian, who lost their only prior match 6-3, 6-7 (4-7), 7-5 in 2002 at Milan.

Federer squandered two break points with forehands beyond the baseline but fired a forehand winner on his third break point to seize a 6-5 edge, then held at love to escape after one hour and 57 minutes.

Agassi, 33, fired five aces and 14 winners in windy weather to triumph in 85 minutes, his 14 unforced errors only half as many as Zabaleta.

Williams, seeded second despite being ranked only 17th, made six double faults and 23 unforced errors but advanced in 79 minutes to face Slovakia’s Daniela Hantuchova in Sunday’s third round.

Safin fired 15 aces and 51 winners past the hometown hero but swatted a forehand wide on Spadea’s second match-point opportunity.

Thai ninth-seeded Paradorn Srichaphan, who could face Spadea in the fourth round, advanced by beating Spanish qualifier Ricardo Mello 6-1, 1-6, 6-3.

American second seed Andy Roddick defeated Slovakian Karol Beck 6-2, 6-4. He will face Sweden’s Jonas Bjorkman next.

Two Dutch seeds were dispatched as number 13 Sjeng Schalken was defaulted for verbal abuse, giving Argentina’s Guillermo Canas a 6-1, 3-0 victory and Aussie wild card Todd Reid ousted number 15 Martin Verkerk 4-6, 6-4, 7-5.