Tennis-Sharapova serve fires up in time for Serena

* Sharapova to meet Williams in quarter-finals * Russian blasts 21 aces to put down Bencic (Adds details, quotes) By Ian Ransom MELBOURNE, Jan 24 (Reuters) - Another day of reckoning looms for Maria Sharapova when she takes on long-time nemesis Serena Williams in the Australian Open quarter-finals but the Russian can at least head into the match with her serve running hot. Runner-up to Williams last year, Sharapova booked a re-match against the American with a hard-fought 7-5 7-5 win over Swiss young gun Belinda Bencic in the early match on Rod Laver Arena on Sunday. Hampered by a number of surgeries on her right shoulder since 2009, Sharapova's serve has rarely proved the difference in tight matches but her 21 aces were vital in fending off 12th seed Bencic, touted as a future grand slam winner. The richly talented 18-year-old had a number of chances to break Sharapova in the second set but the five-times grand slam champion's serve repeatedly sprung her from jail. The 28-year-old now leads the tournament with 52 aces, some distance from Serena's 25, though the American has had to swing her serving arm far less in reaching the quarter-finals. "I feel like that's something I have been working on a lot since I had shoulder surgery," Sharapova told reporters. "I have gone through many different motions to try to find my groove again and something that would hold up over many matches. "That was something that was really difficult to find a rhythm and a balance between keeping the shoulder, not going back so much, because I'm quite loose, to a motion that kind of gave me the chance to play many matches and feel like I was still not able not to be tired after three, four tough matches. "So, yeah, I think I have made a little bit of a change last year in the stance, but I think everyone always tries to improve things here and there. It's not that big of a deal. We always look to add those few percentages." For many observers, the only percentage relevant ahead of the quarter-finals is the win-loss split between Sharapova and her tormentor Williams. Williams has won 90 percent of their matches and is riding a 17-match winning streak against the Russian dating back to 2004. After losing to Williams 6-3 7-6(5) in the final last year, Sharapova said she felt close to breaking the biggest hoodoo in women's tennis but a 6-2 6-4 defeat in the semi-finals of Wimbledon showed the chasm as wide as ever. "It's not like I think about what I can do worse," said Sharapova when asked what she could do to snap the streak. "You're always trying to improve. "It's only going to be tougher, especially against Serena." (Editing by Patrick Johnston)