Angela Merkel's comments about the UK's Brexit stance show the need for Britain to have the "strongest possible hand" in negotiations, says Theresa May.
The PM told a rally in Leeds that 27 EU countries were "lining up to oppose" Britain while her opponents were "trying to disrupt" Brexit talks.
Germany's chancellor said some Britons had "illusions" trade talks could take place alongside Brexit negotiations.
Labour says the Tories have a "rigid and reckless" approach to Brexit talks.
The UK is due to leave the EU by March 2019, once formal negotiations have been completed.
The government has repeatedly said it wants the terms of the separation - including demands for any exit fee - to be dealt with at the same time as a new trading relationship is negotiated.
But EU leaders have resisted this, and on Thursday Mrs Merkel told German MPs it would be "a waste of time" to maintain illusions that the two sets of negotiations could be held simultaneously.
She also said the UK could not maintain the rights it has as an EU member.
"All 27 EU countries and the EU institutions agree about that," she told the Bundestag, Germany's lower house of parliament.
Speaking on a campaign visit to Leeds, Mrs May repeated her claim that an increased Conservative majority was needed to strengthen her Brexit negotiating hand.
"We've seen from Chancellor Merkel today, we've heard her comments today. We've seen that actually there will be times when these negotiations are going to get tough.
"Yet our opponents are already trying to disrupt them at the same time as 27 other European countries are lining up to oppose us. And that can only mean one thing. It will mean uncertainty. It will mean the risk."
'Doing great job'
She urged party supporters not to be complacent and said the polls - which indicate a sizeable Conservative lead - could be wrong, as they had been in 2015. "Every single vote counts," she said.
She urged Labour supporters to vote "in the national interest" to "strengthen my hand when I negotiate with the prime ministers, presidents and chancellors of Europe in the months ahead".
Mrs May also backed Boris Johnson saying he was "doing a great job as foreign secretary". Pressed on his comments earlier that the UK could join the US in another military strike on Syria, possibly without a vote in Parliament, she said that was a "hypothetical issue" as there were no plans for another strike.
The prime minister said: "What Boris has been doing and has been doing very well, is working diplomatically with the international community, with the G7 and with others as well. Because I think now is the time for us to try and find an opportunity to see a way through to a political transition away from [Syrian president] Assad."
The Conservatives are hoping to take seats from Labour on 8 June in areas which voted to leave the EU, including the Midlands, the north-east and north-west of England and across Wales.
While Leeds voted narrowly to remain in the EU, the wider Yorkshire and Humber region as a whole backed Brexit.
Labour set out its approach to Brexit on Tuesday, saying it would scrap Mrs May's plans and unilaterally guarantee the rights of EU residents before talks start.
Shadow Brexit secretary Sir Keir Starmer said Labour wanted a deal which prioritises jobs and workers' rights.
The party's national elections chairman Andrew Gwynne said: "Theresa May is going to extraordinary lengths to blinker the British public and make this election about anything other than her record in government.
"The people of Leeds won't be fooled: the only party of working people is the Labour Party."
And Liberal Democrat leader Tim Farron accused Mrs May of "posturing to try and win votes at home instead of building bridges abroad with our allies in Europe".
"It is shameless opportunism that has nothing to do with May's hand in the Brexit talks and everything to with attempting a naked power grab."