CHARLESTON, S.C. — Last year, unseeded Naomi Osaka made a run to the third round here at the Volvo Car Open.

This year, the number-10 seed—coming off a victory in Indian Wells and a second-round finish in Miami—returns to the round of 16 with a straight-sets win over Laura Siegemund of Germany that was anything but routine.

Osaka got off to a quick start, taking a 3-0 lead. After a visit from her coach, Siegemund— playing in her first tour event in nearly a year after suffering a devastating knee injury—got on the board in the next game, and she was eventually able to recover the early break. Serving at 3-4, though, Siegemund lost the momentum after Osaka blasted a forehand winner on break point.

Down 3-5 now and facing set point, Siegemund hit a drop volley that looked like a sure winner, but Osaka was able to track it for a touch shot into the ad box to clinch the opener.

After an off-court break, Siegemund returned and promptly lost serve. Behind two aces and another big forehand, Osaka consolidated to take a 2-0 lead. Siegemund was under pressure on her serve in the next game, hitting two double faults and missing a drop shot. However, an error from Osaka and two massive serves got her out of trouble.

The two players exchanged holds over the next three games, but Siegemund was able to level the match at 4-all after breaking Osaka. The German held to 5-4, taking the lead for the first time in the match.

With the tension building, Osaka called for her coach at the changeover, and as the players returned to the court, the world No. 24 hit off-speed serves on the first two points to draw errors from Siegemund and get out to an early lead in the game.

After consecutive holds from both players, Siegemund reached set point on Osaka’s serve. Osaka fought it off, hitting an ace down the T to push the match into a tiebreak.

The Indian Wells champion went up a quick mini-break to take a 3-0 lead. Siegemund earned the break back, but Osaka struck again to go up 4-2. Osaka earned her first match at 6-5 as she was able to strike a winner after another drop shot from Siegemund, but was unable to capitalize. A double fault from Siegemund gave Osaka another match point at 7-6, but the German fought it off and won the next two points to go up 8-7.

However, another double-fault from Siegemund allowed Osaka to level the breaker. On the next two points, Osaka blasted a forehand winner, then an ace down the T to open her 2018 clay-court campaign with a victory.

Osaka will face fifth-seed Julia Goerges in the next round after the German battled past American Kristi Ahn in a third-set tiebreak.