Three-time Roland Garros champion Serena Williams staved off a stern challenge from Irina-Camelia Begu in their opening-round clash, ultimately prevailing 7-6(6), 6-2 to book a spot in the second round in Paris.

In a historic first-ever night session on Court Philippe Chatrier, the 23-time Grand Slam champion from the United States had to withstand two set points during a first-set comeback by World No.74 Begu, before finally overcoming the Romanian in an hour and 42 minutes.

"I have to say it was pretty cool to be able to play the first night session ever here at Roland Garros," Williams told the press, following her win. "That was something I thoroughly enjoyed."

Making her 19th appearance at the clay-court major, No.7 seed Williams improved to 77-1 in Grand Slam first-round matches as she obtained her second win in two meetings against former Top 25 player Begu.

Former World No.1 Williams converted five of her eight break points on the day. The American fired 27 winners to Begu's 18 as she moved past the Romanian, whose career-best performance at Roland Garros is a fourth-round showing in 2016.

Williams will now face another crafty Romanian in the second round: former Top 20 player Mihaela Buzarnescu. Left-handed Buzarnescu, who is back on the comeback trail after persistent injuries, defeated Dutchwoman Arantxa Rus, 7-5, 7-5, in her opener on Monday.

After an early exchange of breaks, it was Williams who held the upper hand in the opening frame, slamming a winner crosscourt to break for 4-2, then firing back-to-back aces from deuce in the subsequent game to notch a commanding 5-2 lead. But Begu pulled herself back into contention, forcing errors with sturdy groundstrokes from both sides to get level at 5-5.

Begu then used another error-forcing forehand to break Williams again, collecting a fourth straight game to take a stunning 6-5 lead. But powerful returns by Williams got her out of that jam as she broke Begu right back to line up a decisive first-set tiebreak.

The breaker went back-and-forth up to 4-4, where Begu followed up a strong return with a stellar serve to lead 6-4 and hold two set points. But Williams fended off both -- the second with a heavy second serve to set up a forehand winner. At 6-6, Williams unleashed two more excellent forehands to sweep the next two rallies, ultimately gritting out the final four points of the tiebreak.

"I'm happy that I was able to save those moments," Williams said. "Did not want to lose that first set. I wasn't thinking at that moment. I was just thinking to get that ball out the air, because I've been hitting some good swing volleys in practice."

With a difficult first set behind her, Williams had things more her way in the second, powering to a break in the opening game, then holding firm in a four-deuce tussle to retain a 4-2 lead. Williams eased home from there, finishing up the victory with a superlative serve that was returned long by Begu.

Earlier on Monday evening, Ukrainian teenager Marta Kostyuk pulled off a career-best upset in the first round of Roland Garros, eliminating 2016 champion and No.12 seed Garbiñe Muguruza of Spain, 6-1, 6-4.

"I was so ready and so prepared," Kostyuk told the press, after the match. "My aggressive game today, I’m really glad it worked. I want to keep on going with this game."

The 18-year-old notched her first win over a Top 20 opponent with the 89-minute victory. Kostyuk moved into the second round of the clay-court major for the first time, in her second appearance at the event.

81st-ranked Kostyuk had been 0-4 against the Top 20 previously in her career, but she was able to attain her career-best win by ranking on Court Simonne Mathieu as the sun began to set, firing 16 winners to Muguruza's nine, and breaking serve four times.

Photo by WTA/Jimmie48

Two-time Grand Slam titlist Muguruza struggled with an medical time-out midway through the match, ultimately falling in the opening round of Roland Garros for the very first time in her career. The former champion has had a stellar season up to this point, coming into Paris with a 23-7 win-loss record.

Kostyuk, though, was unbowed by Muguruza's sparkling resume, and came out firing in the first set. A slice backhand broke open a rally to give Kostyuk an early 2-0 lead, and the Ukrainian extended her advantage to 5-1 by forcing more errors from the Spaniard.

The opening set was closer than the scoreline suggested, running 44 minutes, but Kostyuk was able to clinch it after a long backhand return from Muguruza.

After an off-court medical time-out by Muguruza, the Spaniard returned to court with a determined look, and powered her way to an early break for 3-1. But Kostyuk struck back immediately, and after a love hold, she reached parity again at 3-3.

Kostyuk moved ahead by a break in the next game, after Muguruza misfired on a forehand wide. Muguruza superbly fended off a match point at 5-3 with a winning backhand into the corner, but Kostyuk closed out her milestone victory at 5-4 with an angled passing winner.

"I’m really, really happy with my performance today," Kostyuk said. "I’m just trying to focus on the next one. I have two days off and I’m going to try and work again. The work doesn’t stop, so let’s go!"

No.10 seed Belinda Bencic, though, did advance to the second round, easing past Nadia Podoroska, 6-0, 6-3 in just an hour and eight minutes.

Podoroska made a stunning run at Stade Roland Garros just last year, becoming the first qualifier to reach the women's semifinals. However, Bencic quickly prevented the Argentine from pulling off that feat again.

In their first meeting, the Swiss player slammed 22 winners in the clash, while allowing Podoroska to hit just four of her own. Bencic broke Podoroska five times to sweep to the win.

Bencic, who missed last year's Roland Garros with an arm injury, will face a challenging unseeded opponent in the second round: World No.37 Daria Kasatkina, who defeated Japan's Misaki Doi, 6-3, 5-7, 6-3.