World No.1 Roger Federer beat Uzbekistan’s Denis Istomin to reach the quarterfinals of the men’s singles tennis competitions of the London Olympics here on Wednesday.

Federer eventually beat Istomin 7-5, 6-3 after coming under pressure in the first set where he was almost broken by the world No. 35.

In doubles, however, Federer and Stanislas Wawrinka, the defending gold medallists, lost to Jonathan Erlich and Andy Ram of Israel 1-6, 7-6(5), 6-3.

Meanwhile, playing Olympic tennis twice a day proved too much for Venus Williams. Her workload at Wimbledon will now be a little lighter.

Venus squandered a lead in each set and lost her third-round match to No. 7-seeded Angelique Kerber of Germany 7-6(5), 7-6(5).

Venus, seeking a record fourth gold medal in Olympic tennis, still has a shot with her sister Serena in doubles, where they’re in the quarterfinals.

Also reaching the final eight was Serena Williams, who hit 12 aces and repeatedly rocketed her groundstrokes past No. 13-seeded Vera Zvonareva to win 6-1, 6-0.

“I was just playing unbelievable,” Serena said.

“I was nervous going into the match and I didn’t speak to anyone and I had a bad practice. I had no idea I would play like this.”

Men’s No. 2 Novak Djokovic hit 16 aces and came from behind to beat Lleyton Hewitt of Australia 4-6, 7-5, 6-1. Djokovic next plays No. 5 Jo-Wilfried Tsonga of France, who beat Feliciano Lopez of Spain 7-6(5), 6-4 less than 24 hours after winning a 48-game third set.

Four-time Grand Slam champion Kim Clijsters, playing in her first Olympics weeks before she retires, beat former No. 1 Ana Ivanovic of Serbia 6-3, 6-4.

The results: Third round: Women: Angelique Kerber (Ger) bt Venus Williams (U.S.) 7-6(5), 7-6(5); Victoria Azarenka (Blr) bt Nadia Petrova (Rus) 7-6(6), 6-4; Petra Kvitova (Cze) bt Flavia Pennetta (Ita) 6-3, 6-0; Maria Kirilenko (Rus) bt Julia Goerges (Ger) 7-6(5), 6-3; Kim Clijsters (Bel) bt Ana Ivanovic (Srb) 6-3, 6-4; Serena Williams (U.S.) bt Vera Zvonareva (Rus) 6-1, 6-0.

Men: Roger Federer (Sui) bt Denis Istomin (Uzb) 7-5, 6-3; Novak Djokovic (Srb) bt Lleyton Hewitt (Aus) 4-6, 7-5, 6-1; Jo-Wilfried Tsonga (Fra) bt Feliciano Lopez (Esp) 7-6(5), 6-4; Andy Murray (G.Br) bt Marcos Baghdatis (Cyp) 4-6, 6-1, 6-4; Juan Martin del Potro (Arg) bt Gilles Simon (Fra) 6-1, 4-6, 6-3; Nicolas Almagro (Esp) bt Steve Darcis (Bel) 7-5, 6-3.

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