Daniela Hantuchova was upset in the first round of the Zurich Open on Monday, losing in straight sets to Maria Kirilenko.
Hantuchova, the eighth seed, was beaten 7-5 6-2 to exit a tournament in the first round for the second time in her last three outings.
Kirilenko advanced to a second-round meeting with either Denmark's Caroline Wozniacki or Italy's Francesca Schiavone.
Victoria Azarenka of Belarus is also through to the second round after beating France's Amelie Mauresmo 6-2 6-1.
She will face seventh-seeded Anna Chakvetadze of Russia or a qualifier next.
Jelena Jankovic enters this tournament looking to claim her fourth consecutive title in as many weeks.
Jankovic, the world No.1, is currently on a 12-match winning streak and is fresh off Sunday's 6-2 6-4 victory over Russia's Vera Zvonareva in the final of the Kremlin Cup. That victory came on the heels of titles at Beijing and Stuttgart.
Sunday's victory also strengthened the Serb's grip at the top of the rankings.
Jankovic overtook Serena Williams as the world's top player last Monday when the rankings were released.
Jankovic, along with the top four seeds, received a first-round bye and will open her campaign against either Russian Nadia Petrova or Italy's Flavia Pennetta later this week.
The second seed is Ana Ivanovic, whose recent fortunes on tour stand in stark contrast to Jankovic. Ivanovic has struggled since her victory at Roland Garros in June.
In fact, after falling to China's Zheng Jie in the third round at Wimbledon, the 21-year-old has won just three matches, none of them back-to-back, and suffered four early losses.
Ivanovic opens later this week against either France's Marion Bartoli or Hungary's Agnes Szavay.
Also getting underway later this week is the third-seeded Williams and No.4 Zvonareva.
Williams will open against Aliza Bondarenko, who defeated Frenchwoman Alize Cornet, 1-6 6-3 7-6 (8-6) on Monday.
Tennis.com Zurich Open, Switzerland Results Day 1
Singles – First Round
Maria Kirilenko (Russia) def. 8-Daniela Hantuchova (Slovakia) 7-5 6-2
Victoria Azarenka (Belarus) def. Amelie Mauresmo (France) 6-2 6-1
Alona Bondarenko (Ukraine) def. Alize Cornet (France) 1-6 6-3 7-6 (8-6)