NEWPORT BEACH, CA, USA -- The first day of the main draw at the inaugural Oracle Challenger Series event in Newport Beach closed with a shock, as qualifier Mayo Hibi of Japan upset No.2 seed Alison Riske of the United States, 6-3, 6-4, and advanced to the second round on Monday.

World No.232 Hibi had to survive two rounds of qualifying to enter the main draw of the WTA 125K Series event, but once she got there, she frustrated 84th-ranked Riske with an array of shots, ranging from flat forehand winners to lobs, drop shots, and slice backhands.

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Hibi leapt to a 4-0 lead in the first set, as the unforced errors from Riske's racquet quickly mounted. But the American started to find her range, and she continued to use her powerful game to come forward and put away points at the net. Riske broke Hibi's serve twice in a row to get back on serve at 4-3.

Unfortunately for Riske, she let the momentum slip away. Her backhand down the line, which had improved as the match had progressed, failed her, and gave Hibi two break points to take a 5-3 lead. Hibi converted the first one when a crafty backhand with spin forced a forehand error wide from Riske.

Hibi clinched the set in the next game on her second set point, after Riske misfired long on a service return. Riske's aggressive play kept her in Hibi's service games throughout the set, but she had too many errors on key points, going 2-for-11 on break points in the opener.

By contrast, Hibi converted three of six break points, and won a higher percentage of points on her first and second serve than the heavier-serving Riske did.

In the second set, Hibi kept her fine play rolling, breaking Riske at love in the opening game with some spectacular defense and a killer drop shot which gave her triple break point. Riske broke right back for 1-1 with a service return winner, but the American dropped her serve again in the next game.

The Japanese player ceded her break advantage at 3-2, as Riske's backhand returned to fine form to tie the set at three games apiece. But Hibi would not be denied, breaking Riske once more for a 4-3 lead, as Hibi chased everything down and deftly used her slice backhand in rallies to tempt Riske into errors.

Serving at 5-4, Hibi needed three match points to complete the upset, but finally took the win as another slice backhand forced an error long from Riske after one hour and 36 minutes of play. Hibi ended the match breaking Riske six times in nine chances, while Riske had a whopping 15 break points, but converted only four.

The two other seeded players who contested first-round matches on Monday were also upset at the initial hurdle. South Korea's Jang Su Jeong eliminated No.5 seed Sofia Kenin of the United States, 7-6(6), 6-3, and American Kayla Day ousted 8th-seeded Mariana Duque-Mariño of Colombia, 6-3, 7-6(4).

Four American wild cards were also in opening-round action on Monday. Three of the four were victorious, as Victoria Duval rolled past Japan's Misaki Doi, 6-2, 6-1, Danielle Collins beat Czech qualifer Marie Bouzkova, 6-4, 7-6(4), and Claire Liu defeated Stefanie Voegele of Switzerland, 6-4, 7-5.

The other American wild card, Jacqueline Cako, lost to Russian qualifier Sofya Zhuk, 6-1, 6-3. Additionally, in an all-American first-round encounter, qualifier Amanda Anisimova advanced past Irina Falconi when Falconi retired while trailing 6-1, 3-0.