ROME, Italy - Greek youngster Maria Sakkari proved her clay court mettle on Thursday, winning her second match of the day to reach the Internazionali BNL d'Italia quarterfinals as No.2 seed and Porsche Tennis Grand Prix champion Petra Kvitova was forced to retire with a left calf injury, trailing, 7-5, 5-7, 4-0.

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"Petra is the most difficult player for me to play, in my opinion," Sakkari admitted after the match. "She's left-handed, has a big serve and big shots. You never know what she can come up, some unbelievable shots. I was doubting a little in the beginning, 0-3 down. When Tom [Hill] came on court, he gave me a little courage and I changed tactics. I'm very pleased with how I turned the match around."

The Grand Prix de SAR La Princesse Lalla Meryem champion continued the momentum from winning her first title, ousting 2018 Rome semifinalist and Porsche Tennis Grand Prix runner-up Anett Kontaveit earlier in the day and surging towards another big win when Kvitova could no longer continue after two hours and 15 mintes on Centrale Court.

"In the first match, I've played Anett so many times, and we're such good friends. She's a very dangerous player on all surfaces. I just went on court, played my game and served well, which helped. I think I played quite solid this morning, and quite solid this afternoon!"

Kvitova was also doing double duty after Wednesday's washout, dropping just one game to Kazakh rival Yulia Putintseva and winning the first three games to kick off her clash with Sakkari.

The Greek qualifier soon settled, levelling the set after saving a pair of break points in the sixth game and held steady as she turned the tables on the two-time Wimbledon winner, breaking to grab the opening set.

"This means a lot. It's one of my favorite tournaments to play, and I love spending time here. I just love this tournament; since I was a kid, I would see players at this wonderful facility, so I always wanted to be here too. Playing on Center court is really nice, too!"

Kvitova began the second set in similar fashion to the first, breaking early only to see Sakkari even things up at three games apiece, but this time it was the Australian Open runner-up who held her nerve, forcing a decider as Sakkari served for a tie-break.

Sakkari, who reached the Dongfeng Motor Wuhan Open semifinals back in 2017, quickly reclaimed the initiative in the final set, breaking at love twice to take a double break lead as Kvitova faltered physically, calling a medical time-out at 3-0.

The 29-year-old Czech nonetheless made a valiant final push on the Sakkari serve, holding a break point in the very next game, but ultimately was forced to end the contest after the fourth game.

"I knew that my loss in Madrid wasn't because I played badly; it was because [Suárez Navarro] played well, and I wasn't used to the conditions. I had a few days off at home, came back and played qualies, which was tough. Sometimes it's not bad to have more matches and get used to the conditions."

In all, Sakkari struck 19 winners to 24 unforced errors and converted an impressive five of seven break point opportunities. While Kvitova struck 27 winners of her own, she also made 42 unforced errors - nearly half of which came in the opening set - and won just 63% of her first serve points to Sakkari's 70%.

Standing between and a second Premier 5 semifinal is a resurgent Kristina Mladenovic, who ousted two of 2019's standouts in Belinda Bencic and Ashleigh Barty earlier in the day. 

"She's playing really well and made a great move hiring a very good coach in Sascha Bajin. She has the game, and we all knew that because she was a Top 10 player. You can't get there if you're not good, so it's going to be a battle because we'll both want the win. She has very good shots and she'll fight hard. I'll fight too. It's going to be a very interesting match."