Clijsters never looked in trouble as she got past Jankovic in an hour and 21 minutes, 62 64 - an upset given Clijsters is unseeded and Jankovic was seeded No.18, but to form considering Clijsters had won seven of their eight previous meetings and had a 5-3 third set lead in her only loss against the Serb.
"When I saw I was playing her, on the one hand I was like, 'Whew,' because she's tough, but on the other hand it was like, 'Yay,' because over the years I've had some of my funnest matches against her," Clijsters said afterwards. "They've always been physical matches. In a way I was looking forward to it."
With Jankovic's backboard-like game, Clijsters knew she had to be healthy - and an abdominal scare that caused her to withdraw before her semifinal match at the lead-up tournament in 's-Hertogenbosch was cause for concern.
"I was definitely happy to win today, especially with my last two days after I left Rosmalen," Clijsters said. "There were definitely a few worries, but then it's a great feeling when you work so hard towards something and it pays off. For my trainer, for Sam, Carl, Stefan, for my team, it's nice to be able to share those kinds of emotions. The victory on its own felt really special tonight.
"I believe if I'm healthy and playing my best tennis, I can beat a lot of the top players here. It's a matter of trying to achieve that every match."
But that attitude will only last until the US Open - she will go back into retirement after playing at her three-time conquering grounds in New York.
"In the last year or so there were definitely moments where it was hard and where I thought, 'Should I retire now?' Maybe my body is saying it's enough, and this second career has been good, and maybe I should just accept that. But then after a few days I'd say, 'No, I want to do the rehab and refocus.'
"Unfortunately, when one injury got better, another popped up. That's probably been the most frustrating thing over the last 14 months, is that I felt like i've had really good preparations after my rehabbing and preparing for my next few tournaments, and then towards the end something different happens.
"That's the frustating part - I feel like tennis-wise I've played some of my best tennis in the last year and a half or so, but unfortunately I haven't been able to show that in tournaments because of all the injuries that came up."
How does this farewell tour of sorts compare to her first time retiring? "It's easier this time because I'm a little bit older and I understand the emotions better. In that way it's easier; but probably a little bit more emotional this time.
"The first time I was ready to retire, I was convinced. I was like, 'Okay, that's it. Tennis is over.' I think this time is definitely going to be it, so I take everything in. Even when I'm practicing on one of the practice courts, I take it in."
Clijsters and Jankovic weren't the only No.1s in action on Day 1 - current No.1 Maria Sharapova also took the court and won (read more about her match here) while former No.1 Venus Williams was beaten (read more here).
There were some other first round surprises, with No.16 seed Flavia Pennetta falling to Italian qualifier Camila Giorgi, 64 63; No.27-seeded Daniela Hantuchova losing to American Jamie Hampton, 64 76(1); and No.29 seed Monica Niculescu losing to France's Stéphanie Foretz Gacon, 64 36 63.
Agnieszka Radwanska, Sam Stosur, Angelique Kerber, Li Na, Sabine Lisicki, Maria Kirilenko, Nadia Petrova, Petra Cetkovska and Peng Shuai all notched wins for the seeded contingent. Kerber's 64 61 win over Lucie Hradecka was particularly meaningful - it was her 41st match win of the year, which leads the WTA (Radwanska is next on the list with her 39th on Monday).