TOKYO (Kyodo) -- Japan's Moyuka Uchijima battled her way into the second round of the Pan Pacific Open tennis tournament on Tuesday as she edged 15-year-old Mika Stojsavljevic of Britain 6-4, 6(7)-7, 7-6(6).

World No. 57 Uchijima is the host nation's highest-ranked player in the absence of No. 56 Naomi Osaka, who withdrew from the WTA 500 event with lower back and abdominal injuries.

The 23-year-old Uchijima was tested throughout by this year's U.S. Open girls' singles winner but prevailed in the 2-hour, 51-minute marathon at Tokyo's Ariake Colosseum.

"She was a really young opponent at 15, and I've never played someone so much younger than me," said Uchijima, whose mother is Malaysian and father Japanese. "I didn't want to lose out mentally and was determined to play it tough in all three sets."

Both players struggled with their first serves in the rain-interrupted opening set, breaking each other twice each before Uchijima claimed it with her third break, sealed by the 633rd-ranked youngster's double fault.

Uchijima began holding her serve in the second set, but so did the hard-hitting Stojsavljevic, who broke first in the seventh game. Another double fault led to Uchijima's break in the 10th, but Stojsavljevic edged the tiebreak to force the final set.

Playing deep groundstrokes from both sides and returning tenaciously, Stojsavljevic wore down Uchijima in the deciding set and won a break in the fourth game. Uchijima showed her grit by breaking back before tying at 5-5 and forcing another tiebreak.

After Uchijima spurned two match points from 6-4 in the tiebreak, a crisp forehand return down the line and an over-hit backhand from Stojsavljevic sealed it.

"She had many good serves and shots, and I don't know how I won it, to be honest, but this is one that I can build on," Uchijima said. "I've never won in this tournament and tensed up a bit, but I am really happy to have got one today."

Two other Japanese remain in the tournament after both notching their first WTA tour wins. Sayaka Ishii downed compatriot Sara Saito 6-1, 6-1 in a repeat of the result from this month's national championship final, while 29-year-old Kyoka Okamura beat Hailey Baptiste of the United States 7-6(4), 6-3.

(By Morichika Nakamoto)