STUTTGART, Germany - The 11th instalment of the rivalry between Victoria Azarenka and Vera Zvonareva heated up again after an eight-year gap in the first round of the Porsche Tennis Grand Prix, with the Belarusian triumphing 7-5 6-4 in one hour and 44 minutes to level the head-to-head at 5-5.

The last time the pair had met, both were Top 8 players vying for victory in the semifinals of the 2011 WTA Finals in Istanbul - with Azarenka ultimately taking the spoils 6-3, 6-2. Much has happened since then: both are now mothers of one, and today's clash was between two unseeded players ranked outside the Top 50 - indeed, Zvonareva had only gained entry to the main draw as a lucky loser following Garbiñe Muguruza's late withdrawal.

Nonetheless, the two storied competitors put on a fine display of tennis, full of beautifully clean ball-striking and tightly contested to the end, with Azarenka finishing with 29 winners and Zvonareva with 19. Azarenka's ambitious schedule has already involved a flight from the other side of the world, where she competed in Belarus's narrow Fed Cup semifinal loss to Australia at the weekend, this week - and indeed, an opening slew of errors meant that she had to fend off two break points in her opening service game.

Afterwards, the former World No.1 admitted to jetlag. "I have no idea right now of the time or what's happening," she told the press. "The flight from Australia was so long I felt like it was never going to end."

But the 29-year-old soon shook off any lingering effects of jetlag to play her role in a pulsating encounter. Relentlessly pumping the ball around the court and working both depth and angle impeccably, World No.61 Azarenka was single-minded in drawing the short ball and lethal on the finish, and broke for 4-3.

"It was a huge challenge for me today and I'm very happy with the way I handled it," she said. "I was not really expecting too much from myself today in terms of tennis, because I literally played [only] 30 minutes on clay, but I did expect to give my best effort."

Zvonareva, who had won the first eight sets of the rivalry between 2007 and 2009, rose to the challenge with superb defence, pinpoint lobs and sweetly struck backhand winners that recalled her years at the top of the game. Indeed, when Azarenka threw in a loose service game for Zvonareva to level at 4-4, the momentum seemed to be back with the Russian.

But at the climax of the opening stanza, it was the two-time Australian Open champion who moved up a gear, hitting through Zvonareva's counterpunching with ferocious returns to take four straight games from 5-5, breaking twice to put herself in the driver's seat up a set and a break.

World No.84 Zvonareva spent much of the second set valiantly clinging on to the match - and it made for a terrific spectacle as the 34-year-old conjured up impossible passes and clutch serves to stave off a break point in the third game, capture the break back in the fourth and save a further two break points in the seventh.

With the set teetering on the edge and errors creeping back into Azarenka's game, the former World No.1 needed one last push to avoid the rigors of a deciding set - and she found it in style, coming up with some delicate volleys at key moments as she finally broke the Zvonareva serve again for 5-4. There was to be no further twist in the tale as the Monterrey runner-up served out to 30, capturing her first match point as Zvonareva went long with a forehand.

"I wasn't really thinking of getting into a third set," said Azarenka afterwards. "I was just really trying to stay one point at a time because if I start to think too much ahead, I would lose my focus - and to keep my focus today was not easy, obviously, with the circumstances."

Up next for Azarenka is another stern challenge in the form of No.4 seed and defending champion Karolina Pliskova, with a spot in the quarterfinals at stake.