Kim Clijsters made her return to the WTA at the Chicago Fall Tennis Classic on Monday, but in her first match of 2021, the former World No.1 from Belgium lost in her opening-round clash to Hsieh Su-wei of Chinese Taipei.

In the first meeting between the two veterans, Hsieh outlasted four-time Grand Slam champion Clijsters, 6-3, 5-7, 6-3, in a 2-hour and 18-minute tussle on Stadium Court in Chicago.

More from Day 1: Yastremska, Kudermetova advance; Hontama stuns Garcia on WTA debut in Chicago

"We all know she’s a very great, great, great player," Hsieh said on court, following her win. "Outside the court she’s very nice, very polite, very kind, I’ve always liked her a lot. I’m very excited to play her; I’m lucky I didn’t play her ten years ago!"

Following her 2012 retirement, unranked Clijsters is still seeking her first match-win since coming back to WTA action last season, having gone 0-3 in tournament play in 2020. After knee surgery at the end of last year and a bout with COVID-19 this year, Clijsters made her season debut but fell short against Hsieh.

Hsieh bests Clijsters in Belgian's 2021 debut: Chicago Highlights

2021 Chicago 500

Hsieh is also searching for a recovery in form. She reached her first Grand Slam singles quarterfinal at the Australian Open at the start of this year, but has failed to win back-to-back matches at tour level since.

Against Clijsters, Hsieh let 13 of her 20 break points fall by the wayside, but her seven service breaks were still enough to hold on against Clijsters. The Belgian did come back from breaks down in both the second and third sets, but dropped the final two games of the match.

"Obviously, it was exciting to go back out there after a long period of recovery," Clijsters said to the press. "Super excited to be here and to start my match.

"The match, you know, some good things and bad things, inconsistency. Physically being able to get through these matches without big concerns, that was the main goal. I came close today, but I still have a good feeling. I’ve made progression, and I think that’s the most important thing. What I’m looking at is that I’m improving overall, and that’s the positive thing."

Clijsters got off to a hot start, using ferocious forehands to notch the first three games of the match. But Hsieh went on a tear from there, winning eight games in a row to lead by a set and a break at 6-3, 2-0.

Photo by WTA/Jimmie48

However, the aggressive-minded Clijsters pulled herself back into the match and punched a volley winner to garner a crucial break for 6-5 in the second set. Serving for the set, Clijsters saved two break points before firing a forehand crosscourt winner to level the affair.

After falling behind a break in the final set, Clijsters clawed back on serve with a break for 4-3. But Hsieh gritted her way through the next game, drawing errors from the Belgian to break again and serve for the match at 5-3. Hsieh erased two break points in that game before capturing victory after a wide Clijsters miscue.

A challenging test awaits Hsieh in the second round if she wants to claim a second win in a row: World No.16 Ons Jabeur, the sixth seed from Tunisia. It will be the first meeting between the two players, both of whom have an arsenal of crafty shots at their disposal.

Collins keeps winning: No.10 seed Danielle Collins also won her first-round tilt on Monday. The American eased past another Belgian, former Top 20 player Kirsten Flipkens 6-4, 6-2 in 1 hour and 21 minutes. 

World No.28 Collins won nearly 80 percent of her first-service points to steer past 2013 Wimbledon semifinalist Flipkens and book her spot in the second round. Collins will next face Misaki Doi after the Japanese player defeated Marie Bouzkova of the Czech Republic, 6-3, 6-7(4), 6-3.

Collins continues a breakthrough summer, where she won her first WTA singles title on the clay courts on Palermo, then followed up with her second title just two weeks later on the hardcourts of San Jose. Collins has won 15 of her last 18 matches.

Vondrousova rolls: Olympic silver medalist Marketa Vondrousova of the Czech Republic also grabbed a straight-set first-round win, as she dispatched 2021 Wimbledon quarterfinalist Ajla Tomljanovic of Australia, 6-2, 6-1.

Only three spots in the rankings separate World No.41 Vondrousova and 44th-ranked Tomljanovic, but Vondrousova dominated on the day after just over an hour of play, leveling their head-to-head at 2-2.

Tomljanovic dropped serve twice in the first set, both times with double faults, including on Vondrousova's fourth set point at 5-2. The Czech had an easy time in the second set as well, and she saved the single break point she faced during the encounter.

In the second round, 2019 Roland Garros finalist Vondrousova will face 2021 Roland Garros finalist Anastasia Pavlyuchenkova for the first time. No.4 seed Pavlyuchenkova is currently back at her career-high ranking of World No.13.

Sasnovich moves past injured Keys: Madison Keys, though, had to retire from her first-round match due to a right shoulder injury. Former World No.7 Keys was trailing Aliaksandra Sasnovich of Belarus, 6-4, 2-0, when she deemed herself unable to continue the match.

Sasnovich moves forward into a second-round clash with No.5 seed Elena Rybakina of Kazakhstan. Rybakina defeated Sasnovich in the first round of last month's US Open.