Serena show again headlines U.S. Open

NEW YORK (Reuters) - The Serena Williams juggernaut rolls into the third round of the U.S. Open on Friday as the American world number one continues her bid to complete a rare calendar grand slam of the coveted singles titles. Aiming to become just the fourth woman to win all four slams in the same year, Williams will take on compatriot Bethanie Mattek-Sands in the first match of the evening at Arthur Ashe Stadium. "I never stop. I keep going and give the best effort that I can," Williams said after improving her 2015 match record to 50-2 with an error-strewn 7-6(5) 6-3 victory over Dutchwoman Kiki Bertens on Wednesday. Williams said the pressure to join Maureen Connolly (1953), Margaret Court (1970) and Steffi Graf (1988) as the only female grand slam winners had affected her a bit against Bertens. "I just got a little nervous," she said. "I've been doing totally fine. I've been completely relaxed, chill. So I'm going to get back into the place that I was and I'll be fine again." Also in action on Friday will be Serena's older sister Venus, a twice U.S. Open champion who faces a formidable challenge in the shape of rising Swiss talent Belinda Bencic, one of only two players to beat Serena this season. Defending men's champion Marin Cilic of Croatia seeks to keep his run going in a third-round encounter with Kazakhstan's Mikhail Kukushkin while top-seeded Serb Novak Djokovic takes on Italian Andreas Seppi. Djokovic, who won the U.S. Open in 2011, is the reigning Australian and Wimbledon champion and trying to claim his third grand slam singles title of the year. (Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Frank Pingue)