Top seed Petra Kvitova has shaken off her Australian Open hangover to beat Belarusian Victoria Azarenka in the WTA event in St. Petersburg but not before the former world No.1 gave the Czech a scare.
Kvitova, the defending champion, showed she was still feeling the effects of Saturday's defeat to Naomi Osaka in the Australian Open final as she lost her first two service games before she prevailed 6-2 7-6 (7-3).
In the quarter-finals, Kvitova will face eighth-seeded Donna Vekic, who beat lucky loser Veronika Kudermetova 6-4, 6-3.
Maria Sharapova withdrew from the tournament with a right shoulder injury, hours before she was to play Daria Kasatkina in the second round.
"I tried to do everything to be ready to play this week but my right shoulder is still bothering me," she said on the tournament website.
Sharapova had beaten Australia's Daria Gavrilova in the first round on Monday and was aiming to win a WTA event in her native Russia for the first time.
Kasatkina will meet Russian veteran Vera Zvonareva, who upset fifth-seeded Julia Goerges 4-6 6-4 6-4, in the last eight.
Despite her sluggish start on serve, world No.2 Kvitova managed to break back immediately on both occasions before holding for the first time, and then used her powerful groundstrokes to dominate the longer rallies and claim the opener.
With her serve firing, Kvitova charged out of the blocks in the second set, winning the first three games to spark what looked to be a stroll into the quarter-finals and breaking Azarenka's serve again to open up a 5-1 lead.
But the Czech double faulted on match point, allowing Azarenka to break back and mount what was almost a fightback for the ages.
"I don't know why (the double fault) happened. I really wanted to finish it earlier but unfortunately I had to fight in the second set and I did quite well in the tiebreak," Kvitova said courtside after her win.
Azarenka, her confidence restored, pinned Kvitova back behind the baseline with her aggressive tennis, and broke again to level the set at 5-5 and force a tiebreak.
Kvitova hit 40 winners to Azarenka's 14, but she also made 33 unforced errors compared with 17 from her opponent.
The Czech's composure came to the fore in the tiebreak and she closed out the match with a powerful backhand down the line.
