A tearful Osaka said at the prize presentation in Tokyo that she needed a break after her exploits in becoming the first Japanese to win a grand slam title at the US Open in NY earlier this month started to take its toll.
"I'm just happy it didn't go to three sets", said the Czech, who had been taken the distance in her previous three matches. Moments later she delivered the coup de grace with a sixth ace which Osaka barely wafted her racquet at.
Vekic hit back to take the second as Pliskova's serve imploded, the world number eight blotting her copybook with seven double-faults, many of them coming on key points.
Third seed Osaka needed just 71 minutes to complete her 10th straight win after capturing a historic US Open title earlier this month.
Osaka, the runner-up in Tokyo two years ago, climbed to seventh in the rankings after becoming Japan's first grand slam singles champion but she looked lost for ideas against Pliskova.
The newly-crowned US Open champion, who was bidding to become the first Japanese victor of the event since 1995, succumbed 6-4 6-4 in a battle of two big servers.
"It was another long one today", Pliskova said after the match. "She was missing quite a lot today and it was working, so I didn't change anything".
Osaka earned a 6-2, 6-3 victory over the Czech in the quarterfinals of the BNP Paribas Open in March, en route to winning her first career title. "I was a bit nervous because I really wanted to reach the final".
Pliskova soon set her foot after unleashing another brutal drive as Osaka made flat-fault into the net.
"The chances of me playing in the next tournament are very low", she added, only to pull out hours later.
For Pliskova, it was a first tournament victory since Stuttgart in April - and the first since marrying long-time partner Michal Hrdlicka in the summer.
"She has basically every shot and she has a really good serve, so I know it will be a very hard match". "I have my weapons, she also has some".
Klizan, the 2012 champion here, won in 4-6, 6-3, 7-5 in two hours 16 minutes to chalk up his first win over the 33-year-old Wawrinka in their fourth head-to-head meeting.
"I think Donna was playing well and she's beaten a lot of tough players this tournament".
Tennis. Tokyo-2018. The semi-finals.