NEW YORK - Venus Williams didn't mince words in assessing the game shape of her sister and warmup partner on Monday.

"Serena's got power, depth. She knows how to play," Venus said. "Yeah, she's got an awesome game."

The Williams sisters were the headliners on Monday night in the Tie Break Tens tournament in Madison Square Garden, an eight-woman event in which all matches were 10-point tiebreakers, instead of the usual games and sets.

Elina Svitolina of Ukraine dominated the event, winning 17 straight points during victories over Venus and CoCo Vandeweghe before defeating China's Zhang Shuai 10-3 to win $250,000.

Serena won her first match but lost in the semifinals. She was playing singles for the first time this year as she prepares to return to the WTA Tour for the first time since giving birth to her daughter, Alexis Olympia Ohanian Jr, on Sept 1.

The 23-time Grand Slam singles champion would later tell Vogue Magazine in an interview that she developed several small blood clots in her lungs after giving birth.

Serena had played just once this year, losing a Fed Cup doubles match with Venus last month, and hasn't played a singles match on tour since winning the 2017 Australian Open, where she beat Venus in the final.

On Monday, Serena won her opening match against Marion Bartoli of France, the former Wimbledon champion who is ending a four-and-a-half-year retirement, but lost 13-11 to Zhang in the semifinals.

Serena said she was having fun playing again and was excited for the future.

"I think everyone should have high expectations," the 36-year-old said in an on-court interview. "You should always believe in yourself ... even if no one else does."

Her coach certainly does. With Serena returning this week in Indian Wells, California, Patrick Mouratoglou told WTATennis.com he has high expectations for the three remaining Grand Slam tournaments this year.

Mouratoglu said he sees the same determination in practice, but wouldn't know if being a mother had changed her until he saw her competing.

Venus, 37, already knows what to expect.

"I mean, the matches are definitely a good way to know where you are, but obviously she knows how to play," Venus said.

"She's going to bring that instinct on the court with her, so I don't think she's too worried about that."

Tie Break Tens held its first tournament in the US after five events in Europe and Australia. The remainder of the field included Sorana Cirstea of Romania and Daniela Hantuchova of Slovakia, who retired last year.

Svitolina, the highest-ranked player in the field at No 4, continued her strong play to start the season after a winning a tour-high five titles in 2017.

The winner of two tournaments already this year, she won the last seven points to break away from a 3-3 tie against Venus, then swept Vandeweghe 10-0.

"Of course, winning the first tournament of the year was a great start for me," Svitolina said.

"It's always tough to transfer the work we do in the offseason, so this was a good sign that I'm moving the right way and I'm very, very happy that I played good tennis at the beginning of the year ."

Vandeweghe, a New York native, jumped at the chance to play at MSG. Her grandfather, Ernie, and uncle Kiki both played for the NBA's New York Knicks.

"I didn't have to think too long," he said. "Once I was asked by my coach, Pat (Cash), if I was going to be available around that time, I said, 'Yeah, I'll make myself available.' So it didn't take much thinking."

She was on the US team in last month's victory over the Netherlands in North Carolinal.

Venus reached two Grand Slam finals in 2017 and returned to the top five of the rankings.

She competed on Monday night just a few days after the 20th anniversary of her first WTA Tour title in Oklahoma City as a 17-year-old. Many players have taken up the sport since because of the Williams sisters, and Venus said she hoped this format might win over more youngsters.

"I hope that this brings a lot of excitement and more people start playing. It's a great sport. The sport is great for everyone of all ages."

Associated Press