The Gunners centre-back is pleased with the progress his Arsenal team-mate is making this summer, and hailed die Mannschaft's Lukas Podolski after winning his 100th cap
The centre-back, who is in the Germany squad for Euro 2012, also stated his pleasure at the progress of fellow Gunner Alex Oxlade-Chamberlain.
"I'm very pleased for Theo [Walcott] and Alex [Oxlade-Chamberlain]. They were both in and out of the team, but then Theo scored and it was a special moment - it always is when you have to wait for your chance and then play like that," he told reporters.
"He showed that he's a great player, having had a great season for Arsenal. I think it's a great thing when you are not in the starting line-up but are able to show your full strength and take your opportunity. He showed that he is very confident."
The 27-year-old also praised Arsenal-bound Lukas Podolski, who opened the scoring for die Mannschaft against Denmark on his 100th appearance for the country.
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"Sometimes I have a good feeling about players," Mertesacker added. "Lukas [Podolski] showed that he can be a good defensive player but also a good goalscorer - and with his right foot too. It's very good to have him in the squad because he's a striker who is always dangerous in every position on the pitch."
Mertesacker went on to sympathise with fellow centre-back Laurent Koscielny, who has yet to play a minute of action for France at this summer's tournament.
"I'm very happy to see a lot of the players doing well, but I would like to see Laurent play for France too," he added. "I think we are in the same situation - not playing at the moment and having to be ready for our chance."
Germany's quarter-final against Greece in Gdansk on Friday kicks off at 20:45CET.