It was a day of mixed fortunes for former Dubai champions at the Mubadala Abu Dhabi Open on Wednesday as Belinda Bencic booked her place in the quarter-finals but Jelena Ostapenko was sent crashing out. Bencic, who won the Dubai Duty Free Tennis Championships in 2019 and is seeded second this week, kept alive her bid for a UAE double with a 6-4, 7-5 victory over Ukraine's Marta Kostyuk in the first match of the evening session. However, defending Dubai champion Ostapenko was defeated by China's Qinwen Zheng in the first match of the day on Stadium Court, the highly-rated 20-year-old claiming a 7-6, 6-1 win. Having received a bye into the second round, Bencic faced a tricky opponent in Kostyuk and the first set developed into a close battle, with neither player wavering on serve. That was until Bencic created two break point chances in the seventh game, taking the second to lead 4-3. Two solid service holds later, she had claimed the opening set. Bencic took immediate charge of the second set with a break for a 2-0 lead but Kostyuk clawed her way back into the contest, first breaking to level at 2-2 and again for 5-3, giving herself a chance to serve for the set and level the match. Bencic quickly rediscovered her level, though, to reel off four straight games to take the win. "I thought it was a really good level. It was a good first match to get into the tournament," Olympic champion Bencic, 25, said. "I was expecting a tough match against Marta, so I was happy with how things went." On receiving a bye to the second round, Bencic added: "It's an advantage but not really because you might need a first round to get into the tournament and you face a player who's already played a match. It can be a plus and a minus." At the opposite end of the draw, Zheng demonstrated why she is one of the most exciting young talents on the WTA Tour after an impressive win over Ostapenko. Zheng, 20, took a marathon first set following a 22-point tiebreak, before easing through the second to claim a 7-6, 6-1 victory. The Chinese world No 29 will face either Russian top seed Daria Kasatkina or Switzerland's Jil Teichmann in the quarter-finals. It was Zheng's second win over Ostapenko in as many meetings, having beaten the former French Open champion at the US Open last year. "[Ostapenko] plays really aggressive and fast but at the same time I knew that if I stay there she will lose patience," Zheng said. "She's can beat anyone and to beat her you have to run a lot. I'm happy because I did what I had to do to get the win." Veronika Kudermetova, who like Bencic received a bye, opened her campaign with a straight-sets win over Belgium's Elise Mertens as the Russian fourth seed aims to go one better than her last visit to Abu Dhabi. Kudermetova reached the final of the Abu Dhabi WTA Women's Tennis Open in January 2021 and returned to the court with a 6-1, 7-5 victory over former world No 12 Mertens. "It was a very good performance from me," said Kudermetova, who will play either eighth-seeded compatriot Liudmila Samsonova or Czech Barbora Krejcikova in the quarter-finals. "I always like to play in the Middle East and I'm super happy to be back here."