BUCHAREST, Romania -- No.4 seed Petra Martic of Croatia reached the quarterfinals of the BRD Bucharest Open on Thursday, eliminating American qualifier Claire Liu, 6-2, 6-7(5), 6-4 in a tussle that took two hours and 32 minutes.

Martic held three match points in the second set before Liu maneuvered her way into a third set. The Croatian then led 4-0 in the decider, but Liu won four games in a row -- two each before and after a 20-minute rain delay -- to tie the set at 4-4, before Martic won the final two games and took the match on her sixth match point.

"I am happy to win, but I am not happy with the way I played," Martic told the press, after her victory. "I think I was not disciplined enough at the end of the second set, I could have finished it off one and a half hours ago. When you don’t stick to your game this is what happens."

World No.60 Martic won well over half of Liu's service points during the encounter, and had five aces. The duo combined for a large amount of winners -- 35 for Martic and 34 for Liu.

The 18-year-old American acquitted herself well, backing up her Wimbledon performance, where, as the reigning junior titlist, she was the only player to take a set off of eventual champion Angelique Kerber, in the second round. But Liu had 52 unforced errors in the match, 11 more than Martic, which eventually sealed her fate in a close battle.

Martic will face Germany's Laura Siegemund in the last eight, after Siegemund overcame Anna Karolina Schmiedlova of Slovakia during a rain-interrupted match earlier on Thursday, 6-4, 3-6, 6-2.

"[The fans] were great, they were really cheering for me even when I didn’t want to cheer for myself, and it really kept me in this match," said Martic. "It gave me some strength when I was really down, I can just really thank them. I cannot wait to get out there tomorrow."

Martic dominated the early going of the opening set, reaching triple break point at 1-1 by powering through a point with penetrating forehands which Liu dutifully chased down before a final one zoomed past her. The Croatian won the next point to break at love for the 2-1 lead, and broke Liu in her next service game as well for 4-1.

Liu grasped a break back in the following game, saving a game point with a reflex backhand pass that dropped in the corner. The American used that to clinch her only break of the Martic serve in the set, which Martic finally ceded with a backhand drop shot attempt that found the net.

But Martic immediately regained her composure, thumping more forehands to take Liu’s service at love once more, and serve for the set at 5-2. There, Martic reached double set point, and while she double faulted on the first chance, a forehand passing shot by Liu went wide on the second, giving the fourth seed a one-set advantage.

In the second set, Martic claimed what appeared to be a pivotal break to lead 4-3, but Liu fought back to 4-4 with an overhead winner. Martic broke again with solid play to serve for the match at 5-4, but threw in a shocker of a service game, being broken at love after three consecutive unforced errors, and leveling the set once more, at 5-5.

But Liu faltered on serve once more, throwing in a number of miscues to drop serve again. Martic served for the match a second time at 6-5, and held three match points in the game. But Martic hit an forehand unforced error on each chance, and on Liu's third break point of the game, the qualifier hit a forehand winner off an ill-advised drop shot, sending the set to a tiebreak, after six straight breaks of serve.

Liu took advantage of her lifeline, and after reaching 3-3 in the tiebreak via a backhand passing shot winner, the American claimed three points in a row to reach triple set point. Martic’s forehand re-appeared to save the first two on her service, but on the third, a forehand by the Croatian flew long, and Liu found herself level with Martic, at one set apiece.

Despite the missed chances, Martic came out strong in the decider, blasting winners en route to a 3-0 lead, which was punctuated with an ace. Martic broke Liu again for 4-0, and despite handing one break back in the next game, the Croatian seemed to have recovered from her second-set woes.

But Liu consolidated her break, holding for 4-2 with a backhand winner, and then the rain, which had been on-and-off all day, returned. A 20-minute delay occurred, and upon the resumption, Martic lost control of her lead, blowing a 40-15 lead with a flurry of errors, and allowing Liu to break and get back on serve.

Liu held for 4-4 with a backhand passing shot, and then fired groundstroke winners to garner two break points on Martic’s serve. But, with the finish line in sight, Liu dumped two service returns into the net to let Martic pull back to deuce, and the Croatian regrouped and held for 5-4 with a forehand winner.

The American teenager stumbled, and fell behind 0-40 in the next game, giving Martic three more match points, over an hour after she held her first triumvirate. Martic again missed the first two, but on the third, she found her vaunted forehand, and powered a winner past Liu for a difficult victory.